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Question about HLSL matrix multiplication

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I wrote a very simple HLSL shader, but the matrix multiplication confuses me.
// The matrix is in column major
cbuffer Transform : register(b0)
matrix gViewMatrix;
matrix gProjMatrix;
void VS(in float3 pos : POSITION
,in float4 col : COLOR
,out float4 colour : COLOR
,out float4 posH : SV_POSITION
matrix vp = gProjMatrix * gViewMatrix;
// The code below works
posH = mul(gViewMatrix, float4(pos, 1.0));
posH = mul(gProjMatrix, posH);
// The code below doesn't works
//posH = mul(vp, float4(pos, 1.0));
colour = col;
If i multiply the vector twice by mul, the result is right.
But if i use the multiplication result of the matrix, the result is wrong!
Why this will happen?
Edited by 562538332

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