[Bug rtl-optimization/86438] New: [8/9 Regression] wrong code at -Os

[zsojka at seznam dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86438

            Bug ID: 86438
           Summary: [8/9 Regression] wrong code at -Os
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zsojka at seznam dot cz
  Target Milestone: ---
              Host: x86_64-pc-linux-gnu
            Target: x86_64-pc-linux-gnu

Created attachment 44365
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44365&action=edit
reduced testcase

Output:
$ x86_64-pc-linux-gnu-gcc -Os testcase.c
$ ./a.out 
Aborted

Diff between -O2 -> -Os shows:
@@ -77,15 +76,15 @@
 # testcase.c:13:   u32 f = __builtin_sub_overflow_p (d, (u128) d, (u64)0);
        mov     ebx, 0  # _2,
 # testcase.c:12:   u64 d = (g ? 5 : 4);
-       setne   al      #, iftmp.0_9
-       movzx   eax, al # iftmp.0_9, iftmp.0_9
-       add     rax, 4  # iftmp.0_9,
+       cmp     rax, 1  # tmp100,
+       sbb     rax, rax        # iftmp.0_9
+       add     rax, 5  # iftmp.0_9,
 # testcase.c:13:   u32 f = __builtin_sub_overflow_p (d, (u128) d, (u64)0);
        mov     rcx, rax        # _2, iftmp.0_9
        setc    al      #, _11

The - (-O2) never sets CF (thus the last setc always sets al=0).
The + (-Os) set CF if g==0 (thus the last setc sets al=(g==0)).

The __builtin_sub_overflow_p() in fact never overflows, because it does
(u64)d-(u128)d == 0 in all cases.

$ x86_64-pc-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-amd64/bin/x86_64-pc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-262509-checking-yes-rtl-df-extra-nobootstrap-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/9.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++
--enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra
--disable-bootstrap --with-cloog --with-ppl --with-isl
--build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu
--target=x86_64-pc-linux-gnu --with-ld=/usr/bin/x86_64-pc-linux-gnu-ld
--with-as=/usr/bin/x86_64-pc-linux-gnu-as --disable-libstdcxx-pch
--prefix=/repo/gcc-trunk//binary-trunk-262509-checking-yes-rtl-df-extra-nobootstrap-amd64
Thread model: posix
gcc version 9.0.0 20180709 (experimental) (GCC)