http://bugs.llvm.org/show_bug.cgi?id=32690
Bug ID: 32690
Summary: [X86] SBB/AND used instead of XOR/ADC
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: craig.top...@gmail.com
CC: llvm-bugs@lists.llvm.org
On 32-bit targets this C code
unsigned bar(unsigned bitWidth) {
return ((uint64_t)bitWidth + 63) / 64;
}
Produces this assembly output
_bar: ## @bar
## BB#0: ## %entry
movl 4(%esp), %ecx
addl $63, %ecx
sbbl %eax, %eax
andl $1, %eax
shldl $26, %ecx, %eax
retl
As you can see the carry of the addition into eax is implemented using sbb/and
instead of using xor %eax, %eax and an adc.
Using sbb has an unnecessary dependency on the last writer of eax. While the
xor version would not.
--
You are receiving this mail because:
You are on the CC list for the bug._______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
