http://llvm.org/bugs/show_bug.cgi?id=17713
Bug ID: 17713
Summary: missed opportunity to optimize FP division into
multiplication
Product: new-bugs
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Using:
$ ./clang -v
clang version 3.4 (trunk 193529) (llvm/trunk 193527)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
Consider the following reduced code example:
$ cat recip.c
double foo(double x, double y) {
if (y == 2.0) return x/y;
return x;
}
$ ./clang -O3 -fomit-frame-pointer -o - -S recip.c
.section __TEXT,__text,regular,pure_instructions
.section __TEXT,__literal8,8byte_literals
.align 3
LCPI0_0:
.quad 4611686018427387904 ## double 2
.section __TEXT,__text,regular,pure_instructions
.globl _foo
.align 4, 0x90
_foo: ## @foo
.cfi_startproc
## BB#0: ## %entry
ucomisd LCPI0_0(%rip), %xmm1
jne LBB0_2
jp LBB0_2
## BB#1: ## %if.then
divsd %xmm1, %xmm0
LBB0_2: ## %return
ret
It would be great if clang/llvm recognized that the division operation by an
exactly representable FP constant can be reduced to a multiplication. GCC can
do this optimization.
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