https://bugs.llvm.org/show_bug.cgi?id=52392
Bug ID: 52392
Summary: Missed optimization opportunity for loads with a
dynamic index with a known small value range (e.g
0,1).
Product: new-bugs
Version: unspecified
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: t...@google.com
CC: alina.sbir...@gmail.com, htmldevelo...@gmail.com,
llvm-bugs@lists.llvm.org
Loads from an read-only array of known values with a dynamic index which is
known to have only few values (e.g 0 and 1), results in an unnecessary read
from memory.
E.g: https://godbolt.org/z/4r675d6dM
// This keeps 'a' on stack and reads return value from there.
int slow(int num, int v1, int v2) {
int a[2] = {v1, v2};
return a[num&1];
}
//slow (int, int, int):
// mov dword ptr [rsp - 8], esi
// mov dword ptr [rsp - 4], edx
// and edi, 1
// mov eax, dword ptr [rsp + 4*rdi - 8]
// ret
// If we add more code to manually fetch values from 'a', we end up
// with returning one v1/v1 directly and no longer need 'a'.
int fast(int num, int v1, int v2) {
int a[2] = {v1, v2};
int i0 = a[0];
int i1 = a[1];
return (num&1) ? i1:i0;
}
// fast (int, int, int):
// mov eax, esi
// test dil, 1
// cmovne eax, edx
// ret
Being able to get rid of local variables is rather important for performance
on GPUs (NVPTX, AMDGPU) and picking one of the few items in an array is a
fairly common pattern.
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