[Bug target/61441] New: ARM aarch64 fails to quiet signaling NaN

[aurelien at aurel32 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61441

            Bug ID: 61441
           Summary: ARM aarch64 fails to quiet signaling NaN
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aurelien at aurel32 dot net
              Host: aarch64-unknown-linux-gnu
            Target: aarch64-unknown-linux-gnu
             Build: aarch64-unknown-linux-gnu

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.