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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Shrink wrapping not firing on seemingly basic cases? (maybe i misunderstand how it works)"
href="https://bugs.llvm.org/show_bug.cgi?id=33868">33868</a>
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<th>Summary</th>
<td>Shrink wrapping not firing on seemingly basic cases? (maybe i misunderstand how it works)
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>chandlerc@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<div>
<pre>I was digging into sadly unrelated performance issues and noticed a surprising
lack of shrink wrapping for functions with obvious, trivial early-exits.
Here is one example I was able to quickly reproduce:
% cat shrink_wrap.cpp
struct S { long a, b; };
struct C {
S doit() { return {1, 2}; }
};
S compute() {
static C* c = new C();
return c->doit();
}
% ./dev/bin/clang++ -std=c++11 -fno-rtti -fno-exceptions -c -S -o -
shrink_wrap.cpp -O3
.text
.file "shrink_wrap.cpp"
.globl _Z7computev # -- Begin function _Z7computev
.p2align 4, 0x90
.type _Z7computev,@function
_Z7computev: # @_Z7computev
.cfi_startproc
# BB#0: # %entry
pushq %rax
.Lcfi0:
.cfi_def_cfa_offset 16
movb _ZGVZ7computevE1c(%rip), %al
testb %al, %al
jne .LBB0_3
# BB#1: # %init.check
movl $_ZGVZ7computevE1c, %edi
callq __cxa_guard_acquire
testl %eax, %eax
je .LBB0_3
# BB#2: # %init
movl $1, %edi
callq _Znwm
movq %rax, _ZZ7computevE1c(%rip)
movl $_ZGVZ7computevE1c, %edi
callq __cxa_guard_release
.LBB0_3: # %init.end
movl $1, %eax
movl $2, %edx
popq %rcx
retq
.Lfunc_end0:
Why do we need to pushq %rax and popq %rcx in the fast path?</pre>
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