https://llvm.org/bugs/show_bug.cgi?id=31175
Bug ID: 31175
Summary: [AArch64] unnecessary cmp instruction with subtract
and conditional select
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
; ret = (x-y) > 0 ? x-y : 0
define i32 @max(i32 %x, i32 %y) {
%sub = sub nsw i32 %x, %y
%cmp = icmp sgt i32 %sub, 0
%sel = select i1 %cmp, i32 %sub, i32 0
ret i32 %sel
}
Using AArch64 as the target because that shows the problem, but this is likely
a bug for any target that has a subtract that sets comparison flags:
$ ./llc -o - max.ll -mtriple=aarch64
sub w8, w0, w1 <--- could have used 'subs'
cmp w8, #0 <--- and eliminated this cmp
csel w0, w8, wzr, gt
ret
This IR equivalent shows the better codegen:
define i32 @max_better(i32 %x, i32 %y) {
%sub = sub nsw i32 %x, %y
%cmp = icmp sgt i32 %x, %y
%sel = select i1 %cmp, i32 %sub, i32 0
ret i32 %sel
}
$ ./llc -o - max.ll -mtriple=aarch64
subs w8, w0, w1
csel w0, w8, wzr, gt
ret
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