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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 --- - Redundant loads not removed"
href="http://llvm.org/bugs/show_bug.cgi?id=20074">20074</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Redundant loads not removed
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>opt
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ssijaric@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Compiling the following using "clang --target=aarch64-linux-gnu a.c -O3 -c -S"
typedef struct AStruct {
char a;
char b;
char c[10];
} * AS;
extern AS aStruct;
int
main()
{
aStruct->a = 'a';
aStruct->c[2] = 'c';
aStruct->b = 'b';
return 0;
}
results in:
main: // @main
// BB#0: // %entry
adrp x8, aStruct
ldr x9, [x8, :lo12:aStruct] <====
movz w10, #0x61
movz w11, #0x63
strb w10, [x9]
strb w11, [x9, #4]
ldr x8, [x8, :lo12:aStruct] <===== redundant load
movz w9, #0x62
strb w9, [x8, #1]
mov w0, wzr
ret
GVN sees the following:
define i32 @main() #0 {
entry:
%0 = load %struct.AStruct** @aStruct, align 8, !tbaa !1
%a = getelementptr inbounds %struct.AStruct* %0, i64 0, i32 0
store i8 97, i8* %a, align 1, !tbaa !5
%1 = load %struct.AStruct** @aStruct, align 8, !tbaa !1
%arrayidx = getelementptr inbounds %struct.AStruct* %1, i64 0, i32 2, i64 2
store i8 99, i8* %arrayidx, align 1, !tbaa !7
%2 = load %struct.AStruct** @aStruct, align 8, !tbaa !1
%b = getelementptr inbounds %struct.AStruct* %2, i64 0, i32 1
store i8 98, i8* %b, align 1, !tbaa !8
ret i32 0
}
with
!0 = metadata !{metadata !"clang version 3.5.0 "}
!1 = metadata !{metadata !2, metadata !2, i64 0}
!2 = metadata !{metadata !"any pointer", metadata !3, i64 0}
!3 = metadata !{metadata !"omnipotent char", metadata !4, i64 0}
!4 = metadata !{metadata !"Simple C/C++ TBAA"}
!5 = metadata !{metadata !6, metadata !3, i64 0}
!6 = metadata !{metadata !"AStruct", metadata !3, i64 0, metadata !3, i64 1,
metadata !3, i64 2}
!7 = metadata !{metadata !3, metadata !3, i64 0}
!8 = metadata !{metadata !6, metadata !3, i64 1}
where alias analysis tells it that
store i8 99, i8* %arrayidx, align 1, !tbaa !7
clobbers
%2 = load %struct.AStruct** @aStruct, align 8, !tbaa !1
Changing the above example to
typedef struct AStruct {
char a;
char b;
char c;
} * AS;
extern AS aStruct;
int
main()
{
aStruct->a = 'a';
aStruct->c = 'c';
aStruct->b = 'b';
return 0;
}
results in all redundant loads being removed.
Gcc 4.9 removes all redundant loads in the first case.</pre>
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