https://bugs.llvm.org/show_bug.cgi?id=52167
Bug ID: 52167
Summary: Failure to optimize negative abs properly
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
long long abs_1(long long i)
{
return (i > 0) ? -i : i;
}
This is "optimized" by LLVM to `return -llabs(i);`. Instead, it should be
possible to optimize it to avoid the negation by just changing the preceding
compare to the opposite one (as is already done in this example, and that LLVM
currently "optimizes" to have the current output). This is done by GCC, but not
by LLVM.
For comparison, LLVM outputs this on AMD64:
abs_1(long long):
mov rax, rdi
neg rax
cmovl rax, rdi
neg rax
ret
GCC outputs this:
abs_1(long long):
mov rax, rdi
neg rax
cmovns rax, rdi
ret
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