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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 - Byte swap idioms for only part of a wider integer value produce suboptimal code"
href="https://bugs.llvm.org/show_bug.cgi?id=39793">39793</a>
</td>
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<th>Summary</th>
<td>Byte swap idioms for only part of a wider integer value produce suboptimal code
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>7.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Transformation Utilities
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>fabiang@radgametools.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Reduced from a bug reported on llvm-dev.
-
#include <stdint.h>
uint64_t f(uint64_t x)
{
return ((x & 0xff000000ul) >> 3 * 8)
| ((x & 0x00ff0000ul) >> 8)
| ((x & 0x0000ff00ul) << 8)
| ((x & 0x000000fful) << 3 * 8);
}
uint64_t g(uint64_t x)
{
uint32_t y = (uint32_t)x;
return ((y & 0xff000000u) >> 3 * 8)
| ((y & 0x00ff0000u) >> 8)
| ((y & 0x0000ff00u) << 8)
| ((y & 0x000000ffu) << 3 * 8);
}
-
compiled for target x86_64-linux-gnu at -O2 results in this IR:
-
; Function Attrs: norecurse nounwind readnone uwtable
define dso_local i64 @f(i64) local_unnamed_addr #0 {
%2 = lshr i64 %0, 24
%3 = and i64 %2, 255
%4 = lshr i64 %0, 8
%5 = and i64 %4, 65280
%6 = or i64 %3, %5
%7 = shl i64 %0, 8
%8 = and i64 %7, 16711680
%9 = or i64 %6, %8
%10 = shl i64 %0, 24
%11 = and i64 %10, 4278190080
%12 = or i64 %9, %11
ret i64 %12
}
; Function Attrs: nounwind readnone uwtable
define dso_local i64 @g(i64) local_unnamed_addr #1 {
%2 = trunc i64 %0 to i32
%3 = tail call i32 @llvm.bswap.i32(i32 %2)
%4 = zext i32 %3 to i64
ret i64 %4
}
-
Ideally, the output for f should match g.
This would require matching the byte reverse patterns for a narrower sub-type
as well, and inserting trunc/zext around it (as in the code for g). Not sure
how complicated that ends up being.</pre>
</div>
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