https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61839
Bug ID: 61839
Summary: More optimize opportunity
Product: gcc
Version: 4.8.4
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ishiura-compiler at ml dot kwansei.ac.jp
We have two equvalent programs (A) and (B), which only differ by one line
(marked by "// <---HERE"), where (B) simply folds a+972195718 to 972195717,
for local variable a=-1.
Resulting code by "x86_64-unknown-linux-gnu-gcc-4.8.4 -Os -S" is
different; the code for (A) is less optimized than that of (B).
(A)
int main (void)
{
int a = -1;
volatile unsigned b = 1U;
int c = 1;
c = (a + 972195718) >> (1LU <= b); // <---HERE
if (c == 486097858);
else __builtin_abort();
return 0;
}
(B)
int main (void)
{
int a = -1;
volatile unsigned b = 1U;
int c = 1;
c = 972195717 >> (1LU <= b); // <---HERE
if (c == 486097858);
else __builtin_abort();
return 0;
}
+-----------------------------+-----------------------------+
|(A) |(B) |
+-----------------------------+-----------------------------+
|main: |main: |
|.LFB0: |.LFB0: |
| .cfi_startproc | .cfi_startproc |
| subq $24, %rsp | subq $24, %rsp |
| .cfi_def_cfa_offset 32 | .cfi_def_cfa_offset 32 |
| movl $1, 12(%rsp) | movl $1, 12(%rsp) |
| movl 12(%rsp), %eax | movl 12(%rsp), %eax |
| testl %eax, %eax | testl %eax, %eax |
| movl $972195717, %eax | |
| setne %cl | |
| sarl %cl, %eax | |
| cmpl $486097858, %eax | |
| jne .L5 | jne .L2 |
| xorl %eax, %eax | call abort |
| addq $24, %rsp |.L2: |
| .cfi_remember_state | xorl %eax, %eax |
| .cfi_def_cfa_offset 8 | addq $24, %rsp |
| ret | .cfi_def_cfa_offset 8 |
|.L5: | ret |
| .cfi_restore_state | .cfi_endproc |
| call abort | |
| .cfi_endproc | |
+-----------------------------+-----------------------------+
$ x86_64-unknown-linux-gnu-gcc-4.8.4 -v
Using built-in specs.
COLLECT_GCC=x86_64-unknown-linux-gnu-gcc-4.8.4
COLLECT_LTO_WRAPPER=/usr/local/x86_64-tools/gcc-4.8.4/libexec/gcc/x86_64-unknown-linux-gnu/4.8.4/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /home/hassy/gcc/configure
--prefix=/usr/local/x86_64-tools/gcc-4.8.4/ --with-gmp=/usr/local/gmp-5.1.1/
--with-mpfr=/usr/local/mpfr-3.1.2/ --with-mpc=/usr/local/mpc-1.0.1/
--disable-multilib --disable-nls --enable-languages=c
Thread model: posix
gcc version 4.8.4 20140622 (prerelease) (GCC)
