https://llvm.org/bugs/show_bug.cgi?id=28426
Bug ID: 28426
Summary: umulll_overflow with constant powers of two are not
well optimized
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: jmuizel...@mozilla.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
bool k(unsigned long long i) {
unsigned long long result;
return __builtin_umulll_overflow(i, 4, &result);
}
compiles to:
movl $4, %ecx
movq %rdi, %rax
mulq %rcx
seto %al
retq
A better alternative would be this code:
bool k(unsigned long long i)
{
return i >> (64-2);
}
which compiles to:
shrq $62, %rdi
setne %al
retq
This idiom comes from compiled rust code which does an overflow check when
allocating Vecs so it is quite prevalent.
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