[x86] suboptimal codegen for isfinite IR #27538
Description
| Bugzilla Link | 27164 |
| Version | trunk |
| OS | All |
| CC | @hfinkel,@RKSimon |
Extended Description
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.