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OpenGL OpenGL ASM Experiment

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Experimenting with learning x64 asm and how code operates on a low level, and trying to work out where speed is increased in projects when ASM is appropriately used.

My project is x64 with MASM in visual studio 2010

Currently I am trying to replace the basic glClear glLoadIdentity etc with asm to understand how to interface between code and asm.

I'm following the asm examples over at NEHE, but I constantly keep getting an access violation error when I seem to introduce 'push' into the code.

This is the code asm-side:

include gl.inc
include glu.inc

.data
_45d0 equ 40468000h ;45.0
_45d1 equ 0
_01d0 equ 1069128089
_01d1 equ -1717986918 ;0.1
_100d0 equ 1079574528
_100d1 equ 0 ;100.0
_1d0 equ 1072693248
_1d1 equ 0 ;1.0
_05 equ 1056964608 ; 0.5
_1 equ 1065353216 ; 1.0
_m1 equ -1082130432 ;-1.0
_3 equ 1077936128 ; 3.0
_m15 equ -1077936128 ;-1.5
_m6 equ -1061158912 ;-6.0

.code

ASMrender proc
display:

push GL_COLOR_BUFFER_BIT
call glClear
call glLoadIdentity
call glEnd

push _m15
push 0
push _m6
call glTranslatef

xor eax,eax

ret

ASMrender endp
end



Over in C++ I am just doing the standard 'extern "C" void ASMrender
and then calling ASMrender() in the main render loop.

Here's a screenshot of some of the action where it all goes wrong.
2db1fll.png

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Replacing OpenGL calls from C with OpenGL calls from assembler will gain you nothing in terms of performance.

I believe 64-bit windows calling convention for first four floating point arguments requires them to go in SSE2 registers, not on stack: http://msdn.microsof...y/zthk2dkh.aspx
Same thing for integer argument - you should pass argument to glClear on RCX register, not on stack.
You should better write code in C, and check the disassembler how it looks (by putting breakpoint on function call and selecting Dissassembly view from Debug menu - Alt+8). You'll see that Visual Studio will generate pretty optimal code in this case.

Also - you can not call glEnd without glBegin. Edited by Martins Mozeiko

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Also, from http://msdn.microsof...y/ms235286.aspx:

The caller is responsible for allocating space for parameters to the callee, and must always allocate sufficient space for the 4 register parameters, even if the callee doesn’t have that many parameters.
[/quote]

So something like this:

mov rcx, GL_COLOR_BUFFER_BIT ; parameter
sub rsp, 32 ; shadow space for 4 registers
call glClear
add rsp, 32 ; pop register shadows
Edited by Erik Rufelt

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Thanks Rufelt,

I realize this is a somewhat pointless effort in terms of optimizations and yadda yadda. I'd rather just learn how asm works and once I am familiar, in the future I can focus on some decent optimizations with SSE(or so I've heard)

So the asm now looks something like this, however the screen is black, no white triangle:

sub rsp, 32h
mov ecx, GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT
call glClear
add rsp, 32h

call glLoadIdentity

sub rsp, 20h
mov rdx, _m15
mov rcx, 0
mov r8d, _m6
call glTranslatef
add rsp, 20h

sub rsp, 18h
mov edx, GL_TRIANGLES
call glBegin
add rsp, 18h

sub rsp, 20h
mov edx, 0
mov ecx, _1
mov r8d, 0
call glVertex3f
add rsp, 20h

sub rsp, 20h
mov edx, _m1
mov ecx, _m1
mov r8d, 0
call glVertex3f
add rsp, 20h

sub rsp, 20h
mov edx, _1
mov ecx, _m1
mov r8d, 0
call glVertex3f
add rsp, 20h

call glEnd

ret
Edited by dxCUDA

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Cheers for that, Still doesn't seem to be working. All I get is a black screen. I have a feeling it might be the data from _m1, _m15 etc. I'll keep digging

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Well I check the value of _m15 which equated to 0BFC00000h so I created _neg15 dd -1.5f to be more clear on the value and this was also 0BFC00000h .

I'm entirely certain that everything is working accordingly, as if I just chose to contain glClear and glLoadIdentity within the asm function, in conjunction with triangle drawing C++ side, and it works. So it must be incorrect data parsed in asm to the gl procedures.

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glTranslatef arguments are floats. First 4 float arguments of function are passed in SSE2 registers (not rdx/rcx/r8d) as described in the link I and Erik posted above.

And if all you want is to optimize using SSE instructions, then there is no need to do assembly. You simply can use intrinsic functions. It will greatly simplify your life and will give compiler more chance to optimize the code better (inlining & other stuff). Also advantage will be that same code will work for 32-bit target - no need to write assembly twice (for 32 and 64-bit).
http://msdn.microsof...y/y0dh78ez.aspx

And I'll repeat myself. To easier spot mistake in your assembly for such simple code - write the same code in C, and inspect generated assembly (press Alt+8 while debugging) and examine generated assembly code to see the differences from your written assembly. Edited by Martins Mozeiko

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Thanks Mozeiko

That's a good direction to take after this learning exercise. I didn't quite understand from the other post initially about the SEE2 registers, but I do now and it works..

Would asm be viable in a runtime situation whereby we have an if -else statement, we could then do this in asm and avoid the doubled call each frame, as it would remove half the calling?. I could see it being better than C++ in such a situation, or would I still be wrong?

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It works now, thanks for the help, even though the triangle is a little bit weird. Here's the code for any future searches:


include gl.inc
include glu.inc


.data

_neg15 dd -1.5f ;
_neg6 dd -6.0f
_pos1 dd 1.0f
_neg1 dd -1.0f
_20 dd 20.0f


.code

ASMrender proc


sub rsp, 32h
mov ecx, GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT
call glClear
add rsp, 32h

call glLoadIdentity

sub rsp, 32h
movss xmm2, dword ptr [_neg15]
xorps xmm1,xmm1
movss xmm0, dword ptr [_neg1]
call glTranslatef
add rsp, 32h

sub rsp, 32h
mov ecx, GL_TRIANGLES
call glBegin
add rsp, 32h


sub rsp, 32h
xorps xmm2,xmm2
movss xmm1, dword ptr [_pos1]
xorps xmm0,xmm0
call glVertex3f
add rsp, 32h

sub rsp, 32h
movss xmm2, dword ptr [_neg1]
movss xmm1, dword ptr [_neg1]
xorps xmm0,xmm0
call glVertex3f
add rsp, 32h


sub rsp, 32h
movss xmm2, dword ptr [_pos1]
movss xmm1, dword ptr [_neg1]
xorps xmm0,xmm0
call glVertex3f
add rsp, 32h

call glEnd


ret

ASMrender endp
end
Edited by dxCUDA

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Would asm be viable in a runtime situation whereby we have an if -else statement, we could then do this in asm and avoid the doubled call each frame, as it would remove half the calling?. I could see it being better than C++ in such a situation, or would I still be wrong?

It doesn't matter. Any reasonable optimizing C/C++ compiler (including MSVC and GCC) will generate good code for one simple if-else branch. Edited by Martins Mozeiko

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I will never understand this. People wasting time writting things like that in asm. The compiler will do a way better job than you.

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It is interesting to have some understanding of assembler. It's like studying spoken languages, where learning of Latin can be interesting. But most people don't have the time for it.

To optimize, you can almost always get better results spending the effort on algorithms instead.

In the case of OpenGL, trying to optimize while using legacy OpenGL (as OP is doing) is like taking a modern car, replacing the engine with a steam engine, and trying to optimize the steam engine using different qualities of coal.

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Hello dxCUDA. I just noticed your thread. Hey, I have two 64-bit assembly language files in my 3D engine. Most important for you and I, one of them contains a 4x4 matrix multiply function (of f64 AKA double elements), plus a function that transforms my vertices from local to world coordinates. My vertices contain:

1 position
1 zenith vector (normal vector)
1 north vector (tangent vector)
1 east vector (bi-tangent vector)
1 texture-coordinate (just moved from local-vertex to world-vertex)
1 16-bit field of option bits (ditto)
1 texture-ID field (ditto)
1 matrix-ID field (ditto)
1 RGBA color (ditto)

So the function multiplies the input transformation matrix time each position, zenith-vector, north-vector, east-vector in the local-coordinate vertex structure, then stores the result in two world-coordinate vertex structures (one contains f64 == double-precision position/vectors, and the other contains f32 == single precision position/vectors). After the transformation my engine transfers the 32-bit structure to the GPU, then calls glDrawElements().

Anyway, I have not decided whether to make my code open-source yet, but I'm willing to send it to you for education purposes. As I recall, it has quite a few comments at the top about how 64-bit function calls work (where the arguments are, what needs to be preserved, etc). I also have 32-bit versions of the same functions in another file (since I can compile both 32-bit and 64-bit versions of my engine). I also have C code for the matrix multiply, and somewhere in my engine is equivalent C code for the vertex transformation function, so you can compare if you wish.

I was absolutely blown away when I benchmarked these routines. It takes only a few nanoseconds to transform the position and three vectors in each vertex from local to world coordinates, and save it in both 64-bit and 32-bit form (the local-coords input is 64-bit form)... plus transfer the other fields too. And that is only running on one of my 8 cores so far!

PS: I don't think I have a 64-bit version in MASM yet, because my windoze computer is still windoze XP 64-bit edition, which DOES NOT support 16 SIMD registers and DOES NOT support the wider AVX/ymm registers (which hold and process four f64 values at once). What I definitely do have is 64-bit version in GAS (linux syntax), as well as 32-bit versions in GAS and MASM.

If you're interested, let me know. And we can chat on skype if you wish to pick my brain about this topic.

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      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains tutorials, sample applications, asteroids performance benchmark and an example Unity project that uses Diligent Engine in native plugin.
      Atmospheric scattering sample demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

      Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

      Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows, Linux, Android, MacOS, and iOS platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and Metal backend is in the plan.
    • By LifeArtist
      Good Evening,
      I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
      First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
      I am really stucked right now because of the fundamental question:
      Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
      If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on. 
      In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
      Should I treat those debug objects as entities/components?
      For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
      Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
      Regards,
      LifeArtist
    • By QQemka
      Hello. I am coding a small thingy in my spare time. All i want to achieve is to load a heightmap (as the lowest possible walking terrain), some static meshes (elements of the environment) and a dynamic character (meaning i can move, collide with heightmap/static meshes and hold a varying item in a hand ). Got a bunch of questions, or rather problems i can't find solution to myself. Nearly all are deal with graphics/gpu, not the coding part. My c++ is on high enough level.
      Let's go:
      Heightmap - i obviously want it to be textured, size is hardcoded to 256x256 squares. I can't have one huge texture stretched over entire terrain cause every pixel would be enormous. Thats why i decided to use 2 specified textures. First will be a tileset consisting of 16 square tiles (u v range from 0 to 0.25 for first tile and so on) and second a 256x256 buffer with 0-15 value representing index of the tile from tileset for every heigtmap square. Problem is, how do i blend the edges nicely and make some computationally cheap changes so its not obvious there are only 16 tiles? Is it possible to generate such terrain with some existing program?
      Collisions - i want to use bounding sphere and aabb. But should i store them for a model or entity instance? Meaning i have 20 same trees spawned using the same tree model, but every entity got its own transformation (position, scale etc). Storing collision component per instance grats faster access + is precalculated and transformed (takes additional memory, but who cares?), so i stick with this, right? What should i do if object is dynamically rotated? The aabb is no longer aligned and calculating per vertex min/max everytime object rotates/scales is pretty expensive, right?
      Drawing aabb - problem similar to above (storing aabb data per instance or model). This time in my opinion per model is enough since every instance also does not have own vertex buffer but uses the shared one (so 20 trees share reference to one tree model). So rendering aabb is about taking the model's aabb, transforming with instance matrix and voila. What about aabb vertex buffer (this is more of a cosmetic question, just curious, bumped onto it in time of writing this). Is it better to make it as 8 points and index buffer (12 lines), or only 2 vertices with min/max x/y/z and having the shaders dynamically generate 6 other vertices and draw the box? Or maybe there should be just ONE 1x1x1 cube box template moved/scaled per entity?
      What if one model got a diffuse texture and a normal map, and other has only diffuse? Should i pass some bool flag to shader with that info, or just assume that my game supports only diffuse maps without fancy stuff?
      There were several more but i forgot/solved them at time of writing
      Thanks in advance
    • By RenanRR
      Hi All,
      I'm reading the tutorials from learnOpengl site (nice site) and I'm having a question on the camera (https://learnopengl.com/Getting-started/Camera).
      I always saw the camera being manipulated with the lookat, but in tutorial I saw the camera being changed through the MVP arrays, which do not seem to be camera, but rather the scene that changes:
      Vertex Shader:
      #version 330 core layout (location = 0) in vec3 aPos; layout (location = 1) in vec2 aTexCoord; out vec2 TexCoord; uniform mat4 model; uniform mat4 view; uniform mat4 projection; void main() { gl_Position = projection * view * model * vec4(aPos, 1.0f); TexCoord = vec2(aTexCoord.x, aTexCoord.y); } then, the matrix manipulated:
      ..... glm::mat4 projection = glm::perspective(glm::radians(fov), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f); ourShader.setMat4("projection", projection); .... glm::mat4 view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp); ourShader.setMat4("view", view); .... model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f)); ourShader.setMat4("model", model);  
      So, some doubts:
      - Why use it like that?
      - Is it okay to manipulate the camera that way?
      -in this way, are not the vertex's positions that changes instead of the camera?
      - I need to pass MVP to all shaders of object in my scenes ?
       
      What it seems, is that the camera stands still and the scenery that changes...
      it's right?
       
       
      Thank you
       
    • By dpadam450
      Sampling a floating point texture where the alpha channel holds 4-bytes of packed data into the float. I don't know how to cast the raw memory to treat it as an integer so I can perform bit-shifting operations.

      int rgbValue = int(textureSample.w);//4 bytes of data packed as color
      // algorithm might not be correct and endianness might need switching.
      vec3 extractedData = vec3(  rgbValue & 0xFF000000,  (rgbValue << 8) & 0xFF000000, (rgbValue << 16) & 0xFF000000);
      extractedData /= 255.0f;
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