Bug ID: 85376
           Summary: [8 Regression] wrong code with -Og -fno-dce -fgcse
                    -fno-tree-ccp -fno-tree-copy-prop
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot
          Reporter: zsojka at seznam dot cz
  Target Milestone: ---
              Host: x86_64-pc-linux-gnu
            Target: x86_64-pc-linux-gnu

Created attachment 43918
reduced testcase

$ x86_64-pc-linux-gnu-gcc -Og -fno-dce -fgcse -fno-tree-ccp -fno-tree-copy-prop
testcase.c -Wno-psabi
$ ./a.out

At the assembly level:
# testcase.c:16:   q[7] >>= __builtin_add_overflow (0xfffffff0, __builtin_ffs
(n), &s[5]);
        mov     eax, 0  # tmp201,
        bsf     edx, eax        # _28, tmp201
        mov     ecx, -16        # tmp204,
        add     eax, ecx        # tmp203, tmp204
        mov     DWORD PTR [rsp+404], ecx        # s, tmp204
# testcase.c:17:   t = __builtin_ffs (g[7]);
        bsf     eax, DWORD PTR g[rip+28]        # _38, g
        lea     ecx, [rcx+15]   # tmp206,
        cmove   eax, ecx        # tmp206,, _38
        add     eax, 1  # _38,
# testcase.c:18:   e *= __builtin_sub_overflow (o, t, &f);
        mov     ecx, 0  # _41,
        sub     dl, al  # tmp208, _38
        jb      .L7     #,

"sub dl, al" is using edx set in "bsf edx, eax", which is not related (and even
undefined because eax == 0).
Use of ecx is correct, even though it does not look so on the first sight ("lea
ecx, [rcx+15]" just sets ecx=-1 with affecting the flags)

$ x86_64-pc-linux-gnu-gcc -v
Using built-in specs.
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
Thread model: posix
gcc version 8.0.1 20180412 (experimental) (GCC)

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