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<base href="https://bugs.llvm.org/">
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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 - more compact sequence of x86 movs generated when using -g"
href="https://bugs.llvm.org/show_bug.cgi?id=40969">40969</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>more compact sequence of x86 movs generated when using -g
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>greg.bedwell@sony.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Blocks</th>
<td>37728
</td>
</tr></table>
<p>
<div>
<pre>In response to this llvm-dev post:
<a href="http://lists.llvm.org/pipermail/llvm-dev/2019-March/130779.html">http://lists.llvm.org/pipermail/llvm-dev/2019-March/130779.html</a>
Here is an example using the check-cfc tool (
<a href="https://github.com/llvm/llvm-project/tree/master/clang/utils/check_cfc">https://github.com/llvm/llvm-project/tree/master/clang/utils/check_cfc</a> ) to
spot a codegen difference depending on whether -g is specified or not. The
reduced source example is a little bit nasty as there's some dodgy looking
pointer casting in there, but regardless the absence or presence of -g should
not make a difference to generated code.
Here's my check_cfc.cfg file contents:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[Checks]
dash_g_no_change = true
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
and here it is in action:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$ clang -v
Check CFC, checking: dash_g_no_change
clang version 9.0.0 (trunk 355410) (llvm/trunk 355412)
Target: x86_64-scei-ps4
Thread model: posix
InstalledDir: E:\work\upstream-llvm\build-vs2015-ps4-ninja\bin
$ cat 1.cpp
using alpha = float __attribute__((ext_vector_type(4)));
void bravo(alpha charlie) {
unsigned char *delta = (unsigned char *)&charlie;
delta[0] = 0;
volatile alpha echo = charlie;
}
$ clang -c 1.cpp -O2 -g -fno-strict-aliasing --target=x86_64-unknown-unknown
Check CFC, checking: dash_g_no_change
1.cpp Code difference detected with -g
--- c:\tmp\gnochange\example\tmp\tmp_v5akp.o
+++ c:\tmp\gnochange\example\tmp\tmpvhabwu.o
@@ -5,6 +5,6 @@
0x00000001: 48 89 E5 mov rbp,rsp
0x00000004: 0F 29 45 F0 movaps xmmword ptr [rbp-10h],xmm0
0x00000008: C6 45 F0 00 mov byte ptr [rbp-10h],0
-0x0000000C: 0F 28 45 F0 movaps xmm0,xmmword ptr [rbp-10h]
-0x00000010: 0F 29 45 E0 movaps xmmword ptr [rbp-20h],xmm0
-0x00000014: 5D pop rbp
+0x0000000C: 8A 45 F0 mov al,byte ptr [rbp-10h]
+0x0000000F: 88 45 E0 mov byte ptr [rbp-20h],al
+0x00000012: 48 8B 45 F1 mov rax,qword ptr [rbp-0Fh]
*** Diff truncated ***
~~~~~~~~~~~~~~~~~~~~~~~~~~~~</pre>
</div>
</p>
<div id="referenced">
<hr style="border: 1px dashed #969696">
<b>Referenced Bugs:</b>
<ul>
<li>
[<a class="bz_bug_link
bz_status_NEW "
title="NEW - [meta] Make llvm passes debug info invariant"
href="https://bugs.llvm.org/show_bug.cgi?id=37728">Bug 37728</a>] [meta] Make llvm passes debug info invariant
</li>
</ul>
</div>
<br>
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