http://llvm.org/bugs/show_bug.cgi?id=20556
Bug ID: 20556
Summary: fast vector fneg not optimized like fast scalar fneg
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
This is a test for optimizing the "wrong" definition of fneg (one that uses
+0.0 rather than -0.0) when enable-unsafe-fp-math is used.
Using llc built from r214897 on x86-64:
$ cat fast_fneg.ll
define float @fneg(float %x) #0 {
entry:
%sub = fsub float 0.000000e+00, %x
ret float %sub
}
define <4 x float> @fneg4(<4 x float> %x) #0 {
entry:
%sub = fsub <4 x float> zeroinitializer, %x
ret <4 x float> %sub
}
$ ./llc -enable-unsafe-fp-math fast_fneg.ll -o -
.section __TEXT,__text,regular,pure_instructions
.macosx_version_min 13, 3
.section __TEXT,__literal16,16byte_literals
.align 4
LCPI0_0:
.long 2147483648 ## float -0.000000e+00
.long 2147483648 ## float -0.000000e+00
.long 2147483648 ## float -0.000000e+00
.long 2147483648 ## float -0.000000e+00
.section __TEXT,__text,regular,pure_instructions
.globl _fneg
.align 4, 0x90
_fneg: ## @fneg
.cfi_startproc
## BB#0: ## %entry
xorps LCPI0_0(%rip), %xmm0
retq
.cfi_endproc
.globl _fneg4
.align 4, 0x90
_fneg4: ## @fneg4
.cfi_startproc
## BB#0: ## %entry
xorps %xmm1, %xmm1
subps %xmm0, %xmm1
movaps %xmm1, %xmm0
retq
.cfi_endproc
-------------------------------------
We optimized the scalar case to load a constant 0x80000000 to flip the sign bit
and avoid a subtraction. Shouldn't we do the same thing for the vector case?
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