https://llvm.org/bugs/show_bug.cgi?id=27164
Bug ID: 27164
Summary: [x86] suboptimal codegen for isfinite IR
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
define i1 @is_finite(float %x) {
%1 = tail call float @llvm.fabs.f32(float %x)
%2 = fcmp one float %1, 0x7FF0000000000000 ; ordered and not equal
ret i1 %2
}
declare float @llvm.fabs.f32(float)
$ ./llc -o - isfinite.ll
LCPI0_0:
.long 2147483647 ## 0x7fffffff
.long 2147483647 ## 0x7fffffff
.long 2147483647 ## 0x7fffffff
.long 2147483647 ## 0x7fffffff
.section __TEXT,__literal4,4byte_literals
.p2align 2
LCPI0_1:
.long 2139095040 ## float +Inf
.section __TEXT,__text,regular,pure_instructions
.globl _is_finite
.p2align 4, 0x90
_is_finite:
.cfi_startproc
## BB#0:
andps LCPI0_0(%rip), %xmm0
ucomiss LCPI0_1(%rip), %xmm0
setne %al
retq
---------------------------------------------------------------------
Note: 2139095040 = 0x7f800000 (check if the exponent is maxed)
I think this can be reduced to "andnps" with that constant and then ucomiss
against zero (save a load).
Alternatively, we could bring the FP value into an int register and do the
bitwise comparison there. If we have BMI, it could be something like:
movd %xmm0, %eax
andn (load bitmask), %eax, %eax
setne %al ## if all exponent bits were not set, the value is finite
This only needs a scalar load and no explicit compare instruction is needed.
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