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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Missed SROA/DCE optimization opportunity"
href="http://llvm.org/bugs/show_bug.cgi?id=17432">17432</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed SROA/DCE optimization opportunity
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>st@quanttec.com
</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>test1 and test2 in the following C++ code should compile to identical machine
code, but currently don't (with the current trunk version of clang).
struct Result {
int a;
int b;
};
__attribute__((noinline))
void assignZero(int* p) {
*p = 0;
}
inline Result getResult1() {
int a;
assignZero(&a);
Result result;
result.a = a;
result.b = a + 3;
return result;
}
inline Result getResult2() {
Result result;
assignZero(&result.a);
result.b = result.a + 3;
return result;
}
__attribute__ ((noinline))
int test1() {
return getResult1().a;
}
__attribute__ ((noinline))
int test2() {
return getResult2().a;
}
The current trunk version of clang++ generates with "--std=c++11 -O2
-fomit-frame-pointer" the following x64 assembly:
.globl __Z5test1v
.align 4, 0x90
__Z5test1v: ## @_Z5test1v
.cfi_startproc
## BB#0: ## %entry
pushq %rax
Ltmp1:
.cfi_def_cfa_offset 16
leaq 4(%rsp), %rdi
callq __Z10assignZeroPi
movl 4(%rsp), %eax
popq %rdx
ret
.cfi_endproc
.globl __Z5test2v
.align 4, 0x90
__Z5test2v: ## @_Z5test2v
.cfi_startproc
## BB#0: ## %entry
pushq %rax
Ltmp3:
.cfi_def_cfa_offset 16
leaq (%rsp), %rdi
callq __Z10assignZeroPi
movl (%rsp), %eax
addl $3, %eax
movl %eax, 4(%rsp)
movl (%rsp), %eax
popq %rdx
ret
.cfi_endproc</pre>
</div>
</p>
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