https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116723

            Bug ID: 116723
           Summary: performance differs in  -ffinite-loop with only type
                    changing
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bouncy12578 at gmail dot com
  Target Milestone: ---

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
13.2.0-23ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-13
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-libstdcxx-backtrace --enable-gnu-unique-object --disable-vtable-verify
--enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-13-uJ7kn6/gcc-13-13.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-uJ7kn6/gcc-13-13.2.0/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.0 (Ubuntu 13.2.0-23ubuntu4)

$ gcc -O3 -ffinite-loops case1.c - o out1.o && ./out1.o
1
$ gcc -O3 -ffinite-loops case1.c - o out2.o && ./out2.o
(no results, a infinite loop)

case1.c:
#include "stdio.h"
#include "stdint.h"

union U0 {
    int16_t  f0;
    int32_t  f1;
} g;

int func_1(union U0 p)
{
    p.f1 = 0;
    for (p.f0 = 1; (p.f0 != (-18)); p.f0 = (int16_t)(3 - p.f0));
    return 1;
}

int main () {
    printf("%d", func_1(g));
    return 0;
}


case2.c:
#include "stdio.h"
#include "stdint.h"

union U0 {
    uint16_t  f0;
    int32_t  f1;
}g;

int func_1(union U0 p)
{
    p.f1 = 0;
    for (p.f0 = 1; (p.f0 != (-18)); p.f0 = (uint16_t)(3 - p.f0));
    return 1;
}

int main () {
    printf("%d", func_1(g));
    return 0;
}

The change between these two files is line 5 and line 12, from int16_t to
uint16_t.
With -O3 and -ffinite-loops optimization, case1.c outputs result "1", while
case2.c is considered as an infinite loop.

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