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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 --- - constexpr code unnecessarily generated and called"
href="http://llvm.org/bugs/show_bug.cgi?id=15968">15968</a>
</td>
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<tr>
<th>Summary</th>
<td>constexpr code unnecessarily generated and called
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<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>C++11
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hhinnant@apple.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=10497" name="attach_10497" title="date.cpp">attachment 10497</a> <a href="attachment.cgi?id=10497&action=edit" title="date.cpp">[details]</a></span>
date.cpp
I've been working on a constexpr date class that validates its input. If all
of the input is compile-time information, the validation should happen at
compile time.
The enclosed file ends with:
int
main()
{
constexpr ymd_date ymd(year(2013), month(5), day(11));
return ymd.year();
}
Using tip-of-trunk clang, and -O3, the generated assembly for main is:
.section __TEXT,__text,regular,pure_instructions
.globl _main
.align 4, 0x90
_main: ## @main
.cfi_startproc
## BB#0: ## %entry
pushq %rbp
Ltmp2:
.cfi_def_cfa_offset 16
Ltmp3:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp4:
.cfi_def_cfa_register %rbp
movl $2013, %edi ## imm = 0x7DD
movl $5, %esi
movl $11, %edx
callq __ZN8ymd_date16check_invariantsE4year5month3day
movl $2013, %eax ## imm = 0x7DD
popq %rbp
ret
.cfi_endproc
However the compiler should be able to deduce that the call to
__ZN8ymd_date16check_invariantsE4year5month3day is unnecessary because:
1. It is constexpr.
2. It is called in the process of constructing a constexpr.
3. It compiled.
Thus it can't possibly have any run time impact. What I'm looking for is:
.section __TEXT,__text,regular,pure_instructions
.globl _main
.align 4, 0x90
_main: ## @main
.cfi_startproc
## BB#0: ## %entry
pushq %rbp
Ltmp2:
.cfi_def_cfa_offset 16
Ltmp3:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp4:
.cfi_def_cfa_register %rbp
movl $2013, %eax ## imm = 0x7DD
popq %rbp
ret
.cfi_endproc
Which is *all* that gets generated if I change the enclosure to:
constexpr ymd_date ymd(year(2013), month(5), day(11));
int
main()
{
return ymd.year();
}
In the example the programmer can quickly loose track of what input needs to be
validated and which doesn't (because it is compile-time information). And this
seems to me to be one of the things that constexpr could do well.</pre>
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