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Date: Thu, 28 Feb 2019 03:09:54 +1100 (AEDT)
From: Damian McGuckin <damianm@....com.au>
To: musl@...ts.openwall.com
Subject: Re: FE Exception triggered by comparison
Thanks for the feedback.
On Wed, 27 Feb 2019, Alexander Monakov wrote:
> On Thu, 28 Feb 2019, Damian McGuckin wrote:
>
>> I tried the code below, I was just shocked.
>>
>> #include <math.h>
>> #include <stdio.h>
>>
>> main()
>> {
>> double x = 5.0;
>>
>> x -= x, x /= x;
>> printf("what %s\n", isnan(x) ? "yes" : "no!");
>> return(0);
>> }
>>
>> Looking at the assembler, there is a subroutine call to __isnan. Awful!
>
> Hm, no, for x86 with GCC you should not see that: the compiler knows how
> to expand isnan efficiently. Are you perhaps on OS X and the 'gcc'
> command actually invokes Clang/LLVM?
No.
> If not, can you show output of 'gcc -v', command-line flags you used,
> and the assembly you're seeing?
gcc -O3 -S -msse4.2 -mfma mynan.c
Here is the assembler:
.file "mynan.c"
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "yes"
.LC1:
.string "no!"
.LC3:
.string "what %s\n"
.section .text.startup,"ax",@progbits
.p2align 4,,15
.globl main
.type main, @function
main:
.LFB14:
.cfi_startproc
vxorpd %xmm0, %xmm0, %xmm0
subq $8, %rsp
.cfi_def_cfa_offset 16
vdivsd %xmm0, %xmm0, %xmm0
call __isnan
movl $.LC0, %esi
testl %eax, %eax
movl $.LC1, %eax
cmove %rax, %rsi
movl $.LC3, %edi
xorl %eax, %eax
call printf
xorl %eax, %eax
addq $8, %rsp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
.LFE14:
.size main, .-main
.ident "GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-36)"
.section .note.GNU-stack,"",@progbits
Regards - Damian
Pacific Engineering Systems International, 277-279 Broadway, Glebe NSW 2037
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