[Bug rtl-optimization/94873] [8/9/10 Regression] wrong code with -O -fno-merge-constants -fno-split-wide-types -fno-tree-fre

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94873

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-04-30
   Target Milestone|---                         |8.5

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Testcase without printf:
__attribute__((noipa)) void
foo (const char *p, int q)
{
  if (p[0] != '%' || p[1] != '0' || p[2] != '2' || p[3] != 'x' || p[4] != '\0'
|| (unsigned char) q != 0x95)
    __builtin_abort ();
}

int
main ()
{
  char x = ((union { __int128 a; char b[16]; }){ .a =
0xF4409395252B9560}).b[1];
  for (unsigned i = 0; i < sizeof (x); i++)
    foo ("%02x", i[(volatile unsigned char *) &x]);
  return 0;
}

The difference (before that commit and with that) is:
--- pr94873.s1  2020-04-30 14:31:25.834578823 +0200
+++ pr94873.s2  2020-04-30 14:32:46.452456073 +0200
@@ -19,8 +19,11 @@ main:
        ret
        .size   main, .-main
        .section        .rodata
-       .align  3
+       .align  4
        .set    .LANCHOR0,. + 0
+.LC0:
+       .xword  -846514461162629792
+       .xword  0
 .LC1:
        .string "%02x"
        .ident  "GCC: (GNU) 8.0.1 20180315 (experimental)"
i.e. the unused constant somehow made it into the .rodata section.