<html>
<head>
<base href="http://llvm.org/bugs/" />
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - More optimize opportunity"
href="http://llvm.org/bugs/show_bug.cgi?id=22527">22527</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>More optimize opportunity
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ishiura-compiler@ml.kwansei.ac.jp
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>We would just like to report that GCC (4.9.2, -O3) does stronger
optimization than Clang (3.7.0, -O3) on the following C code.
Evaluation of the arith expression (involving volatile variable a)
is optimized out by GCC.
int main (void)
{
volatile unsigned int a = 1U;
unsigned long c = (a + 0x80000000LU) * 0xFFFFFFFFFFFFFFFELU;
if (c == 0xFFFFFFFEFFFFFFFELU) ;
else __builtin_abort();
return 0;
}
LLVM/Clang-3.7.0 "-O3 -S"
main: # @main
.cfi_startproc
# BB#0: # %entry
pushq %rax
.Ltmp0:
.cfi_def_cfa_offset 16
movl $1, 4(%rsp)
movl 4(%rsp), %eax
movq $-2147483648, %rcx # imm = 0xFFFFFFFF80000000
subq %rax, %rcx
movabsq $9223372036854775807, %rax # imm = 0x7FFFFFFFFFFFFFFF
andq %rcx, %rax
movabsq $9223372034707292159, %rcx # imm = 0x7FFFFFFF7FFFFFFF
cmpq %rcx, %rax
jne .LBB0_2
# BB#1: # %if.end
xorl %eax, %eax
popq %rdx
retq
.LBB0_2: # %if.else
callq abort
.Ltmp1:
.size main, .Ltmp1-main
.cfi_endproc
.ident "clang version 3.7.0 (trunk 228120)"
.section ".note.GNU-stack","",@progbits
GCC-4.9.2 "-O3 -S"
main:
.LFB0:
.cfi_startproc
subq $24, %rsp
.cfi_def_cfa_offset 32
movl $1, 12(%rsp)
movl 12(%rsp), %eax
cmpl $1, %eax
jne .L5
xorl %eax, %eax
addq $24, %rsp
.cfi_remember_state
.cfi_def_cfa_offset 8
ret
.L5:
.cfi_restore_state
call abort
.cfi_endproc
.LFE0:
.size main, .-main
.section .text.unlikely
.LCOLDE0:
.section .text.startup
.LHOTE0:
.ident "GCC: (GNU) 4.9.2"
.section .note.GNU-stack,"",@progbits</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>