https://bugs.llvm.org/show_bug.cgi?id=42571
Bug ID: 42571
Summary: [x86] forming subo intrinsic thwarts codegen
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: craig.top...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
Forking this example from bug 42314 because it's not exactly an "isPowerOf2"
pattern:
int foo(int x, int y) {
return x ? (x & (x - 1)) : y;
}
Or as optimized IR (if we should create ctpop from this, then this isn't
strictly an x86 bug):
define i32 @foo(i32 %x, i32 %y) {
%tobool = icmp eq i32 %x, 0
%sub = add nsw i32 %x, -1
%and = and i32 %sub, %x
%cond = select i1 %tobool, i32 %y, i32 %and
ret i32 %cond
}
CGP will transform that to use llvm.usub.with.overflow.i32, and we get:
movl %edi, %eax
subl $1, %eax
andl %edi, %eax
cmpl $1, %edi
cmovbl %esi, %eax
retq
For generic x86, this seems better
leal -1(%rdi), %eax
andl %edi, %eax
testl %edi, %edi
cmove %esi, %eax
And with BMI ('blsr'):
blsr %edi, %eax
testl %edi, %edi
cmove %esi, %eax
https://godbolt.org/z/Wmgyy9
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