[Bug c++/93674] GCC eliminates conditions it should not, when strict-enums is on

[gbuella at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93674

--- Comment #6 from Gábor Buella <gbuella at gmail dot com> ---
(In reply to Jonathan Wakely from comment #4)
> I can't reproduce this with GCC 9, only 8.

$ cat code.cc
enum some_enum { x = 1000 };

void sink(some_enum);

void func()
{
        for (int i = 0; i < 3; ++i) {
                int j = 3;
                while (j >= 0) { // turned into an uncoditional jmp by GCC
                        // Note: (i + j) is always non-negative here
                        // Actually, (i + j) is always of the values
                        // given when defining the type above.
                        sink((some_enum)(i + j));
                        --j;
                }
        }
}

$ g++-9 -Wall -Wextra -pedantic -std=c++14 code.cc   -fstrict-enums  -S -o
/dev/stdout -fno-dwarf2-cfi-asm -O3  | head -20
        .file   "code.cc"
        .text
        .p2align 4
        .globl  _Z4funcv
        .type   _Z4funcv, @function
_Z4funcv:
.LFB0:
        endbr64
        pushq   %rbx
.LCFI0:
        movl    $3, %ebx
        .p2align 4,,10
        .p2align 3
.L2:
        movl    %ebx, %edi
        subl    $1, %ebx
        call    _Z4sink9some_enum@PLT
        jmp     .L2
.LFE0:
        .size   _Z4funcv, .-_Z4funcv

$ g++-9 --version
g++-9 (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.