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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Functionattrs incorrectly marks argument as readnone"
href="http://llvm.org/bugs/show_bug.cgi?id=19842">19842</a>
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<th>Summary</th>
<td>Functionattrs incorrectly marks argument as readnone
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>rob.lougher.llvm@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>Running the following IR through opt -functionattrs will incorrectly mark
%store's %p as readnone. The bug is due to incorrectly handling the copy of
the pointer made by @get_pntr, because @get_pntr itself is marked readnone.
define i32* @get_pntr(i32* %p) nounwind uwtable {
entry:
ret i32* %p
}
define void @store(i32* %p) noinline nounwind uwtable {
entry:
%call = call i32* @get_pntr(i32* %p)
store i32 10, i32* %call, align 4
ret void
}
This can be seen with the following C testcase. The bug leads to GVN
incorrectly removing the return load:
======== test.c ===========
int *get_pntr(int *p) {
return p;
}
__attribute__((noinline))
void store(int *p) {
int *p2 = get_pntr(p);
*p2 = 10;
}
int test() {
int i;
store(&i);
return i;
}
-----------------------------
Compile in two steps as follows:
clang -O1 -emit-llvm test.c -c -o test.bc
opt -basicaa -inline -functionattrs -gvn -S test.bc
We get the following IR:
--------------------------------------------------
define i32* @get_pntr(i32* readnone %p) {
entry:
ret i32* %p
}
define void @store(i32* nocapture readnone %p) {
entry:
store i32 10, i32* %p, align 4, !tbaa !1
ret void
}
define i32 @test() {
entry:
%i = alloca i32, align 4
call void @store(i32* %i)
ret i32 undef
}
--------------------------------------------------
GVN removes the load because it believes that the call to %store has no affect
on %i, and as %i has just been allocated, the loaded value is undefined.</pre>
</div>
</p>
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