Hello
I noticed a strange behaviour of the DMD compiler when it has to
call a function with float arguments.
I build with the flags "-mcpu=avx2 -O -m64" under windows 64
bits using "DMD32 D Compiler v2.090.1-dirty"
I have the following function :
float mul_add(float a, float b, float c); //Return a * b + c
When I try to call it :
float f = d_mul_add(1.0, 2.0, 3.0);
I tested with other functions with float parameters, and there is
the same problem.
Then the following instructions are generated :
//Loads the values, as it can be expected
vmovss xmm2,dword [rel 0x64830]
vmovss xmm1,dword [rel 0x64834]
vmovss xmm0,dword [rel 0x64838]
//Why ?
movq r8,xmm2
movq rdx,xmm1
movq rcx,xmm0
//
call 0x400 //0x400 is where the mul_add function is located
My questions are :
- Is there a reason why the registers xmm0/1/2 are saved in
rcx/rdx/r8 before calling ? The calling convention specifies that
the floating point parameters have to be put in xmm registers,
and not GPR, unless you are using your own calling convention.
- Why is it done using non-avx instructions ? Mixing AVX and
non-AVX instructions may impact the speed greatly.
Any idea ? Thank you in advance.
