[Bug c/68431] New: Regression in GCC-6.0.0's optimizer

["ishiura-compiler at ml dot kwansei.ac.jp" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68431

            Bug ID: 68431
           Summary: Regression in GCC-6.0.0's optimizer
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ishiura-compiler at ml dot kwansei.ac.jp
  Target Milestone: ---

ïWe compiled a program (A.c) by GCC-5.2.1 and GCC-6.0.0 with -O3 option.
GCC-5.2.1 performed better optimization than GCC-6.0.0.
Note that "unsigned int" is 32 bit and "long long int" is 64 bit.

(A.c)
unsigned int x = 1;
int main (void)
{
        long long int a = -2LL;
        int t = 1<=(a/x);
        if (t != 0) { __builtin_abort(); }

        return 0;
}

+---------------------------------------+--------------------------------------+
|   gcc-5.2.1s (gcc-5.2.1 A.c -O3 -S)   |  gcc-6.0.0.s (gcc-6.0.0 A.c -O3 -S) 
|
+---------------------------------------+--------------------------------------+
|main:                                  |main:                                
|
|.LFB0:                                 |.LFB0:                               
|
|        .cfi_startproc                 |        .cfi_startproc               
|
|                                       |        movl    x(%rip), %ecx        
|
|                                       |        movq    $-2, %rax            
|
|                                       |        cqto                         
|
|                                       |        idivq   %rcx                 
|
|                                       |        testq   %rax, %rax           
|
|                                       |        jg      .L7                  
|
|        xorl    %eax, %eax             |        xorl    %eax, %eax           
|
|        ret                            |        ret                          
|
|                                       |.L7:                                 
|
|                                       |        pushq   %rax                 
|
|                                       |        .cfi_def_cfa_offset 16       
|
|                                       |        call    abort                
|
|        .cfi_endproc                   |        .cfi_endproc                 
|
|.LFE0:                                 |.LFE0:                               
|
|        .size   main, .-main           |        .size   main, .-main         
|
|        .section        .text.unlikely |                                     
|
|.LCOLDE0:                              |                                     
|
|        .section        .text.startup  |                                     
|
|.LHOTE0:                               |                                     
|
|        .globl  x                      |        .globl  x                    
|
|        .data                          |        .data                        
|
|        .align 4                       |        .align 4                     
|
|        .size   x, 4                   |        .size   x, 4                 
|
|x:                                     |x:                                   
|
|        .long   1                      |        .long   1                    
|
|        .ident  "GCC: (Ubuntu 5.2.1 ...|        .ident  "GCC: (GNU) 6.0.0 
...|
|        .section        .note.GNU-  ...|        .section        .note.GNU-
...|
+---------------------------------------+--------------------------------------+



$ gcc-6.0 -v
Using built-in specs.
COLLECT_GCC=gcc-6.0
COLLECT_LTO_WRAPPER=/home/iwatsuji/opt/gcc-6.0.0/libexec/gcc/x86_64-pc-linux-gnu/6.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
configured with: ../gcc/configure --prefix=/home/iwatsuji/opt/gcc-6.0.0
--program-suffix=-6.0 --disable-multilib --enable-languages=c
Thread model: posix
gcc versin 6.0.0 20151119 (experimental) (GCC) 

$ gcc-5 -v
Using built-in specs.
COLLECT_GCC=gcc-5
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
5.2.1-23ubuntu1~14.04.2' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-5 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=gcc4-compatible --disable-libstdcxx-dual-abi
--enable-gnu-unique-object --disable-vtable-verify --enable-libmpx
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.2.1 20151031 (Ubuntu 5.2.1-23ubuntu1~14.04.2)