[Bug target/61494] New: -fsignaling-nans not taken into account for x - 0.0

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61494

            Bug ID: 61494
           Summary: -fsignaling-nans not taken into account for x - 0.0
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net

Created attachment 32932
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=32932&action=edit
testcase

On x86_64, x - 0.0 is optimized to x in the asm code when compiling with -O
-fsignaling-nans. This yields an incorrect behavior when x is a signaling NaN.
Note that -fdump-tree-all shows that -fsignaling-nans is taken into account
when the usual optimization is considered; even transf.c.170t.optimized still
has "_2 = x_1(D) - 0.0;" as expected (I suppose).

On PowerPC, the same x - 0.0 optimization is done with GCC 4.7.2 at least.

I've attached a program that triggers the bug (it also does other tests).

$ gcc -O transf.c -o transf -fsignaling-nans -lm
$ ./transf 0x7ff0000000000001
x = [0x7ff0000000000001] = nan (signaling)
Error for x_sub_zero in FE_TONEAREST:
  y0 = [0x7ff0000000000001] = nan (signaling)
  y1 = [0x7ff8000000000001] = nan
Error for x_sub_zero in FE_TOWARDZERO:
  y0 = [0x7ff0000000000001] = nan (signaling)
  y1 = [0x7ff8000000000001] = nan
Error for x_sub_zero in FE_DOWNWARD:
  y0 = [0x7ff0000000000001] = nan (signaling)
  y1 = [0x7ff8000000000001] = nan
Error for x_sub_zero in FE_UPWARD:
  y0 = [0x7ff0000000000001] = nan (signaling)
  y1 = [0x7ff8000000000001] = nan