MSVC compiler generates mov ecx, ecx that looks useless [duplicate] - c++

Question

This question already has answers here:
Is mov %esi, %esi a no-op or not on x86-64?
(2 answers)
Why did GCC generate mov %eax,%eax and what does it mean?
(1 answer)
Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?
(4 answers)
Closed 3 years ago.
I have some C++ code that is being compiled to the following assembly using MSVC compiler v14.24:
00007FF798252D4C vmulsd xmm1,xmm1,xmm7
00007FF798252D50 vcvttsd2si rcx,xmm1
00007FF798252D55 vmulsd xmm1,xmm7,mmword ptr [rbx+28h]
00007FF798252D5A mov ecx,ecx
00007FF798252D5C imul rdx,rcx,0BB8h
00007FF798252D63 vcvttsd2si rcx,xmm1
00007FF798252D68 mov ecx,ecx
00007FF798252D6A add rdx,rcx
00007FF798252D6D add rdx,rdx
00007FF798252D70 cmp byte ptr [r14+rdx*8+8],0
00007FF798252D76 je applyActionMovements+15Dh (07FF798252D8Dh)
As you can see, the compiler added two
mov ecx,ecx
instructions that don't make any sense to me, because they move data from and to the same register.
Is there something that I'm missing?
Here is a small Godbolt reproducer: https://godbolt.org/z/UFo2qe
int arr[4000][3000];
inline int foo(double a, double b) {
return arr[static_cast<unsigned int>(a * 100)][static_cast<unsigned int>(b * 100)];
}
int bar(double a, double b) {
if (foo(a, b)) {
return 0;
}
return 1;
}

Answer 1

That's an inefficient way to zero-extend ECX into RCX. More efficient would be mov into a different register so mov-elimination could work.
Duplicates of:
Why did GCC generate mov %eax,%eax and what does it mean?
Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?
But your specific test-case needs zero-extension for a slightly non-obvious reason:
x86 only has conversion between FP and signed integers (until AVX512). FP -> unsigned int is efficiently possible on x86-64 by doing FP -> int64_t and then taking the low 32 bits as unsigned int.
This is what this sequence is doing:
vcvttsd2si rcx,xmm1 ; double -> int64_t, unsigned int result in ECX
mov ecx,ecx ; zero-extend to promote unsigned to ptrdiff_t for indexing
add rdx,rcx ; 64-bit integer math on the zero-extended result