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------------------------------------------------------------------------
On 2014-06-07T10:44:28+00:00 Aurelien Jarno wrote:

Consider the following code:

#define _GNU_SOURCE
#include <stdio.h>
#include <math.h>

int main (void)
{
  float sNaN = __builtin_nansf ("");
  double x = (double) sNaN;
  return issignaling(x);
}

It correctly returns 0 at -O0 optimisation level, but returns 1 at -O1
and -O2 optimisation levels.

Here is the generated assembly code at -O0:
0000000000400630 <main>:
  400630:       a9be7bfd        stp     x29, x30, [sp,#-32]!
  400634:       910003fd        mov     x29, sp
  400638:       18000160        ldr     w0, 400664 <main+0x34>
  40063c:       b9001fa0        str     w0, [x29,#28]
  400640:       b9401fa0        ldr     w0, [x29,#28]
  400644:       1e270000        fmov    s0, w0
  400648:       1e22c000        fcvt    d0, s0
  40064c:       9e660000        fmov    x0, d0
  400650:       f9000ba0        str     x0, [x29,#16]
  400654:       fd400ba0        ldr     d0, [x29,#16]
  400658:       97ffff8e        bl      400490 <__issignaling@plt>
  40065c:       a8c27bfd        ldp     x29, x30, [sp],#32
  400660:       d65f03c0        ret
  400664:       7fa00000        .word   0x7fa00000

Here is the generated assembly code at -O1:
0000000000400630 <main>:
  400630:       a9bf7bfd        stp     x29, x30, [sp,#-16]!
  400634:       910003fd        mov     x29, sp
  400638:       5c000080        ldr     d0, 400648 <main+0x18>
  40063c:       97ffff95        bl      400490 <__issignaling@plt>
  400640:       a8c17bfd        ldp     x29, x30, [sp],#16
  400644:       d65f03c0        ret
  400648:       00000000        .word   0x00000000
  40064c:       7ff40000        .word   0x7ff40000

As you can see at -O1, the sNaN constant is propagated, and the propagated value is loaded and passed directly to issignaling. Quoting the IEEE Std 754 standard:
"Under default exception handling, any operation signaling an invalid operation exception and for which a floating-point result is to be delivered shall deliver a quiet NaN."

So it looks like the copy propagation is not done correctly, the sNaN
should be silenced in the process.

Reply at:
https://bugs.launchpad.net/ubuntu/+source/gcc-4.8/+bug/1327530/comments/0

------------------------------------------------------------------------
On 2014-08-12T16:33:35+00:00 Ramana-gcc wrote:

I see the same behaviour on x86_64-linux-gnu , there is nothing specific
about this behaviour with respect to the AArch64 port.

I can see this is ccp that causes the propagation in the brief time I
looked at it. What I haven't yet done is audit the options bits to work
out what the issues here.

Reply at:
https://bugs.launchpad.net/ubuntu/+source/gcc-4.8/+bug/1327530/comments/2


** Changed in: gcc
       Status: Unknown => New

** Changed in: gcc
   Importance: Unknown => Medium

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https://bugs.launchpad.net/bugs/1327530

Title:
  [arm64] float to double conversion does not silence sNaN

Status in The GNU Compiler Collection:
  New
Status in Linaro GCC:
  New
Status in “gcc-4.8” package in Ubuntu:
  New
Status in “gcc-4.9” package in Ubuntu:
  New

Bug description:
  see the upstream issue

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