RyuJIT: Optimize "X / POW2_CNS" via cmovns

[EgorBo]

int Test(int x) => x / 4;

Current codegen:

 8BC1 mov eax, ecx
 C1F81F sar eax, 31
 83E003 and eax, 3
 03C1 add eax, ecx
 C1F802 sar eax, 2
 C3 ret 
; Total bytes of code: 14

Expected codegen:

 8D4103 lea eax, [rcx+3]
 85C9 test ecx, ecx
 0F49C1 cmovns eax, ecx
 C1F802 sar eax, 2
 C3 ret 
; Total bytes of code: 12

This micro-peephole-optimization was added recently in LLVM (PR, see BuildSDIVPow2), see godbolt: https://godbolt.org/z/M153rj
My working (ugly) prototype for RyuJIT: EgorBo@9b1d149 (I believe it should be done the other way - I'd introduce a GT_SELECT/GT_CMOV operator so we can later use it for other cmov-based optimizations, e.g. remove branches)

Benchmark

[Benchmark]
public void Test()
{
 for (int i = 0; i < 10000; i++)
 Consume(i / 4 + i / 8 + i / 16 + i / 32);
}

[MethodImpl(MethodImplOptions.NoInlining)]
static void Consume(int x) { }

 | Method | Mean | Error | StdDev |
 |------- |---------:|---------:|---------:|
 master | Test | 19.14 us | 0.017 us | 0.013 us |
prototype | Test | 17.02 us | 0.016 us | 0.013 us | ~11% faster

/cc @AntonLapounov

category:cq
theme:basic-cq
skill-level:beginner
cost:small
impact:small