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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization for ((X ashr C1) & C2) == 0"
href="https://bugs.llvm.org/show_bug.cgi?id=48640">48640</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed optimization for ((X ashr C1) & C2) == 0
</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>
<tr>
<th>OS</th>
<td>Linux
</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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>void foo();
void bad1(int e)
{
if (((e >> 31) & 64) == 0) foo();
}
void bad2(int e)
{
if (((e >> 31) & 64) != 0) foo();
}
void ok1(unsigned e)
{
if (((e >> 31) & 64) == 0) foo();
}
void ok2(unsigned e)
{
if (((e >> 31) & 64) != 0) foo();
}
LLVM:
bad1(int): # @bad1(int)
shr edi, 25
test dil, 64
jne .LBB0_1
jmp foo() # TAILCALL
.LBB0_1:
ret
bad2(int): # @bad2(int)
shr edi, 25
test dil, 64
jne .LBB1_2
ret
.LBB1_2:
jmp foo() # TAILCALL
ok1(unsigned int): # @ok1(unsigned int)
jmp foo() # TAILCALL
ok2(unsigned int): # @ok2(unsigned int)
ret
GCC:
bad1(int):
test edi, edi
jns .L4
ret
.L4:
jmp foo()
bad2(int):
test edi, edi
js .L7
ret
.L7:
jmp foo()
ok1(unsigned int):
jmp foo()
ok2(unsigned int):
ret
<a href="https://godbolt.org/z/x85bf6">https://godbolt.org/z/x85bf6</a>
So..
((X ashr C1) & C2) != 0 to X < 0
..and
((X ashr C1) & C2) == 0 to X >= 0?
define i1 @src(i32 %0) {
%1:
%2 = ashr i32 %0, 31
%3 = and i32 %2, 64
%4 = icmp eq i32 %3, 0
ret i1 %4
}
=>
define i1 @tgt(i32 %0) {
%1:
%r = icmp sge i32 %0, 0
ret i1 %r
}
Transformation seems to be correct!
define i1 @src(i32 %0) {
%1:
%2 = ashr i32 %0, 31
%3 = and i32 %2, 64
%4 = icmp ne i32 %3, 0
ret i1 %4
}
=>
define i1 @tgt(i32 %0) {
%1:
%r = icmp slt i32 %0, 0
ret i1 %r
}
Transformation seems to be correct!</pre>
</div>
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