[Bug target/111165] [13 regression] builtin strchr miscompiles on Debian/x32 with dietlibc

["tg at mirbsd dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111165

--- Comment #5 from Thorsten Glaser <tg at mirbsd dot org> ---
I managed to isolate one specific strchr call changing which causes the
breakage to go away:

    asm volatile("nop"); //401
                                sp = cstrchr(sp, '\0') + 1;
    asm volatile("nop"); //403

where:

#define cstrchr(s,c)    ((const char *)strchr((s), (c)))

        const char *sp;

I inserted NOPs around (see above) and set gdb breakpoints there, and
before/after those breakpoints, the callee-saved registers differ (only showing
callee-saved registers for amd64/x32):

(gdb) info r
rbx            0x0                 0
rbp            0xf7ff790c          4160715020
r12            0x2                 2
r13            0xffffd1b4          4294955444
r14            0xf7ff510c          4160704780
r15            0x0                 0
(gdb) c
(gdb) info r
rbx            0xf7ff790e          4160715022
rbp            0xf7ff790c          4160715020
r12            0xf7ff7912          4160715026
r13            0xffffd1b4          4294955444
r14            0xf7ff510c          4160704780
r15            0x0                 0

Corresponding disassembly of generated code between the NOPs:

=> 0x0040a635 <+1875>:  nop
   0x0040a636 <+1876>:  mov    0x48(%rsp),%ebx
   0x0040a63a <+1880>:  mov    %rbx,%rdi
   0x0040a63d <+1883>:  call   0x42cc6e <strlen>
   0x0040a642 <+1888>:  mov    0x48(%rsp),%ecx
   0x0040a646 <+1892>:  lea    0x1(%rcx,%rax,1),%r12d
   0x0040a64b <+1897>:  nop

or, in readable:

=> 0x0040a635 <+1875>:  nop
   0x0040a636 <+1876>:  mov    ebx,DWORD PTR [rsp+0x48]
   0x0040a63a <+1880>:  mov    rdi,rbx
   0x0040a63d <+1883>:  call   0x42cc6e <strlen>
   0x0040a642 <+1888>:  mov    ecx,DWORD PTR [rsp+0x48]
   0x0040a646 <+1892>:  lea    r12d,[rcx+rax*1+0x1]
   0x0040a64b <+1897>:  nop

OK, huh… no strchr involved here.

For comparison, with -fno-builtin-strchr:

=> 0x0040a632 <+1872>:  nop
   0x0040a633 <+1873>:  mov    ebx,DWORD PTR [rsp+0x48]
   0x0040a637 <+1877>:  xor    esi,esi
   0x0040a639 <+1879>:  mov    rdi,rbx
   0x0040a63c <+1882>:  call   0x42cba4 <strchr>
   0x0040a641 <+1887>:  mov    r14d,eax
   0x0040a644 <+1890>:  inc    eax
   0x0040a646 <+1892>:  mov    DWORD PTR [rsp+0x3c],eax
   0x0040a64a <+1896>:  nop

I’ll dig into strlen in dietlibc/x32 next.