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<base href="https://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 --- - __builtin_assume pessimization"
href="https://llvm.org/bugs/show_bug.cgi?id=25476">25476</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>__builtin_assume pessimization
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>3.7
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>matt@godbolt.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider the code: [c.f. <a href="https://goo.gl/7VB27y">https://goo.gl/7VB27y</a> ]
--
enum class Side { Bid, Ask };
struct Foo { int a; int b; };
int test(Side side, const Foo &foo) {
if (side == Side::Bid) return foo.a;
return foo.b;
}
int test_with_unreachable(Side side, const Foo &foo) {
if (side == Side::Bid) return foo.a;
if (side == Side::Ask) return foo.b;
__builtin_unreachable();
}
int test_with_assume(Side side, const Foo &foo) {
__builtin_assume(side == Side::Bid || side == Side::Ask);
if (side == Side::Bid) return foo.a;
return foo.b;
}
int test_with_assume2(Side side, const Foo &foo) {
if (side == Side::Bid) return foo.a;
__builtin_assume(side == Side::Ask);
return foo.b;
}
--
One might expect the generated code to basically be the same, but the _assume2
case generates a branch instead of the conditional moves emitted elsewhere.
e.g.
--
test(Side, Foo const&): # @test(Side, Foo const&)
lea rax, [rsi + 4]
test edi, edi
cmove rax, rsi
mov eax, dword ptr [rax]
ret
--
vs
--
test_with_assume2(Side, Foo const&): # @test_with_assume2(Side, Foo
const&)
test edi, edi
je .LBB3_2
add rsi, 4
.LBB3_2:
mov eax, dword ptr [rsi]
ret
--
This would appear to be a pessimization.
Arguably given the __builtin_assumption that the Side is either 0 or 1, one
might hope the following code could be generated:
--
mov eax, dword ptr [rsi+edx*4]
ret
--
GCC has a similar bug with __builtin_unreachable
(<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68274">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68274</a>)</pre>
</div>
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