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<body><span class="vcard"><a class="email" href="mailto:hans@chromium.org" title="Hans Wennborg <hans@chromium.org>"> <span class="fn">Hans Wennborg</span></a>
</span> changed
<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - possible bad code generation"
href="https://llvm.org/bugs/show_bug.cgi?id=31614">bug 31614</a>
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<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>What</th>
<th>Removed</th>
<th>Added</th>
</tr>
<tr>
<td style="text-align:right;">Status</td>
<td>NEW
</td>
<td>RESOLVED
</td>
</tr>
<tr>
<td style="text-align:right;">CC</td>
<td>
</td>
<td>ahmed.bougacha@gmail.com
</td>
</tr>
<tr>
<td style="text-align:right;">Resolution</td>
<td>---
</td>
<td>FIXED
</td>
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<b><a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - possible bad code generation"
href="https://llvm.org/bugs/show_bug.cgi?id=31614#c2">Comment # 2</a>
on <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - possible bad code generation"
href="https://llvm.org/bugs/show_bug.cgi?id=31614">bug 31614</a>
from <span class="vcard"><a class="email" href="mailto:hans@chromium.org" title="Hans Wennborg <hans@chromium.org>"> <span class="fn">Hans Wennborg</span></a>
</span></b>
<pre>(In reply to <a href="show_bug.cgi?id=31614#c1">comment #1</a>)
<span class="quote">> Hans, this looks like PR31367
> (<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - X86: Generate smaller code for atomic refcount decrement"
href="show_bug.cgi?id=31367#c8">https://llvm.org/bugs/show_bug.cgi?id=31367#c8</a>). What do you think?</span >
Yes, that's exactly it. The bug was introduced in r265636, i.e. llvm 3.9.
My best idea for working around it would be doing the fetch_add operation in a
noinline function :-/
My fix in r291630, which will be in llvm 4, affects the repro like this:
@@ -19,12 +19,13 @@
_Z13test_functionv: # @_Z13test_functionv
.cfi_startproc
# BB#0: # %entry
+ movl $1, %eax
+ lock xaddq %rax, foo(%rip)
testq %rax, %rax
setns %al
- lock incq foo(%rip)
movl $value_a, %ecx
movl $value_b, %edx
- cmovgq %rcx, %rdx
+ cmovnsq %rcx, %rdx
cmpl $100, (%rdx)
je .LBB1_3
# BB#1:
@@ -37,11 +38,12 @@
movl $foo+16, %eax
cmovneq %rcx, %rax
lock incq (%rax)
+ movl $1, %eax
+ lock xaddq %rax, foo(%rip)
testq %rax, %rax
setns %al
- lock incq foo(%rip)
movl $value_b, %esi
- cmovgq %rdx, %rsi
+ cmovnsq %rdx, %rsi
cmpl $100, (%rsi)
jne .LBB1_2
.LBB1_3: # %if.end.thread</pre>
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