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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 - [InstCombine] ashr disguised as lshr+signmask"
href="https://bugs.llvm.org/show_bug.cgi?id=43705">43705</a>
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<th>Summary</th>
<td>[InstCombine] ashr disguised as lshr+signmask
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</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>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
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<pre>I don't really personally need this, but i just noticed that we don't do this
fold,
so documenting. One might write such a monstrosity because technically,
arithmetic right shift is implementation defined until C++20 (don't know about
C.)
<a href="https://godbolt.org/z/XPn2Dw">https://godbolt.org/z/XPn2Dw</a>
#include <cstdint>
int t0(unsigned x, unsigned y) {
uint32_t bottom = x >> y;
uint32_t top = (-(x >> 31)) << (32 - y);
return top | bottom;
}
int t1(unsigned x, unsigned y) {
uint32_t bottom = x >> y;
uint32_t top = (-(x >> 31)) & (-1U << (32 - y));
return top | bottom;
}
int t2(unsigned x) {
uint32_t bottom = x >> 4;
uint32_t top = (-(x >> 31)) << (32 - 4);
return top | bottom;
}
int t3(unsigned x) {
uint32_t bottom = x >> 4;
uint32_t top = (-(x >> 31)) & (-1UL << (32 - 4));
return top | bottom;
}
These are all should be just arithmetic right-shift:
<a href="https://rise4fun.com/Alive/USf">https://rise4fun.com/Alive/USf</a>
Name: ashr disguised
%3 = lshr i32 %x, %1
%4 = ashr i32 %x, 31
%5 = sub i32 32, %1
%6 = shl i32 %4, %5
%r = or i32 %6, %3
=>
%r = ashr i32 %x, %1
Name: ashr disguised, with mask
%3 = lshr i32 %x, %1
%4 = ashr i32 %x, 31
%5 = sub i32 32, %1
%6 = shl i32 -1, %5
%7 = and i32 %6, %4
%r = or i32 %7, %3
=>
%r = ashr i32 %x, %1
Name: ashr disguised, constants
Pre: C2 u>= C1 && C3 == C2-C1
%2 = lshr i32 %x, C1
%3 = ashr i32 %x, C2
%4 = shl i32 %3, C3
%r = or i32 %4, %2
=>
%r = ashr i32 %x, C1
Name: ashr disguised, constants, with mask
Pre: C2 u>= C1 && countLeadingOnes(C3) == C1 && countTrailingZeros(C3) == 32-C1
%2 = lshr i32 %x, C1
%3 = ashr i32 %x, C2
%4 = and i32 %3, C3
%r = or i32 %4, %2
=>
%r = ashr i32 %x, C1</pre>
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