<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Issue</th>
<td>
<a href=https://github.com/llvm/llvm-project/issues/55123>55123</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>
InstCombinePass makes value of constant variable not available when debugging and InlinerPass on SCC then drops it entirely
</td>
</tr>
<tr>
<th>Labels</th>
<td>
</td>
</tr>
<tr>
<th>Assignees</th>
<td>
</td>
</tr>
<tr>
<th>Reporter</th>
<td>
dcdelia
</td>
</tr>
</table>
<pre>
In this minimized C example, variable `l_141`, defined within the scope of function `a`, is not available on the call site upon calling the externally defined function `test`. This happens at every tested optimization level (`-Og/-O1/-O2/-O3`).
With opt-bisect-limit we found out that there are two passes involved in this information loss: `InstCombinePass on function (a)` and `InlinerPass on SCC (main)`. If we stop the optimization pipeline right after the former, variable `l_141` will be marked as optimized out instead of having the correct value available. If we stop the optimization pipeline right after the latter, variable `l_141` will even disappear from the list retrieved with the frame variables command in lldb.
We tested clang and lldb 14.0.0 commit 116dc70 on x64.
```
$ cat a.c
void a()
{
char b = 0;
short l_141 = 2;
int l_149 = 3, l_151 = 0;
for (; b != 9; b = b + 1)
{
test(l_141, l_149, l_151);
l_141 = 5;
}
}
int main()
{
a();
}
$ cat lib.c
#include <stdio.h>
void test(int l_141, int l_149, int l_151) {
printf("%d %d %d", l_141, l_149, l_151);
}
```
LLDB trace:
```
$ clang -Ox -g a.c lib.c -o opt
$ lldb opt
(lldb) target create "opt"
Current executable set to '/tmp/opt' (x86_64).
(lldb) b 8
Breakpoint 1: 2 locations.
(lldb) r
Process 172 launched: '/tmp/opt' (x86_64)
Process 172 stopped
* thread #1, name = 'opt', stop reason = breakpoint 1.2
frame #0: 0x0000000000400510 opt`main [inlined] a at a.c:8:9
5 int l_149 = 3, l_151 = 0;
6 for (; b != 9; b = b + 1)
7 {
-> 8 test(l_141, l_149, l_151);
9 l_141 = 5;
10 }
11 }
(lldb) frame var
(int) l_151 = 0
(int) l_149 = 3
```
**IR before** `InstCombine`:
```
define dso_local void @a() local_unnamed_addr #0 !dbg !7 {
call void @llvm.dbg.value(metadata i8 0, metadata !11, metadata !DIExpression()), !dbg !18
call void @llvm.dbg.value(metadata i16 2, metadata !13, metadata !DIExpression()), !dbg !18
call void @llvm.dbg.value(metadata i32 3, metadata !15, metadata !DIExpression()), !dbg !18
call void @llvm.dbg.value(metadata i32 0, metadata !17, metadata !DIExpression()), !dbg !18
br label %1, !dbg !19
1: ; preds = %6, %0
%2 = phi i16 [ 2, %0 ], [ 5, %6 ], !dbg !18
%3 = phi i8 [ 0, %0 ], [ %10, %6 ], !dbg !18
call void @llvm.dbg.value(metadata i8 %3, metadata !11, metadata !DIExpression()), !dbg !18
call void @llvm.dbg.value(metadata i16 %2, metadata !13, metadata !DIExpression()), !dbg !18
%4 = sext i8 %3 to i32, !dbg !20
%5 = icmp ne i32 %4, 9, !dbg !23
br i1 %5, label %6, label %11, !dbg !24
6: ; preds = %1
%7 = zext i16 %2 to i32
%8 = call i32 (i32, i32, i32, ...) bitcast (i32 (...)* @test to i32 (i32, i32, i32, ...)*)(i32 %7, i32 3, i32 0), !dbg !25
call void @llvm.dbg.value(metadata i16 5, metadata !13, metadata !DIExpression()), !dbg !18
%9 = add nsw i32 %4, 1, !dbg !27
%10 = trunc i32 %9 to i8, !dbg !28
call void @llvm.dbg.value(metadata i8 %10, metadata !11, metadata !DIExpression()), !dbg !18
br label %1, !dbg !29, !llvm.loop !30
11: ; preds = %1
ret void, !dbg !34
}
declare dso_local i32 @test(...) local_unnamed_addr #1
; Function Attrs: nounwind uwtable
define dso_local i32 @main() local_unnamed_addr #0 !dbg !35 {
call void @a(), !dbg !38
ret i32 0, !dbg !39
}
[ . . .]
!13 = !DILocalVariable(name: "l_141", scope: !7, file: !1, line: 4, type: !14)
```
**IR after** `InstCombine`:
```
define dso_local void @a() local_unnamed_addr #0 !dbg !7 {
call void @llvm.dbg.value(metadata i8 0, metadata !11, metadata !DIExpression()), !dbg !18
call void @llvm.dbg.value(metadata i16 2, metadata !13, metadata !DIExpression()), !dbg !18
call void @llvm.dbg.value(metadata i32 3, metadata !15, metadata !DIExpression()), !dbg !18
call void @llvm.dbg.value(metadata i32 0, metadata !17, metadata !DIExpression()), !dbg !18
br label %1, !dbg !19
1: ; preds = %6, %0
%2 = phi i32 [ 2, %0 ], [ 5, %6 ]
%3 = phi i32 [ 0, %0 ], [ %9, %6 ], !dbg !18
call void @llvm.dbg.value(metadata i8 undef, metadata !11, metadata !DIExpression()), !dbg !18
call void @llvm.dbg.value(metadata i16 undef, metadata !13, metadata !DIExpression()), !dbg !18
%4 = shl i32 %3, 24, !dbg !20
%5 = icmp eq i32 %4, 150994944, !dbg !23
br i1 %5, label %10, label %6, !dbg !24
6: ; preds = %1
%7 = ashr exact i32 %4, 24, !dbg !20
%8 = call i32 (i32, i32, i32, ...) bitcast (i32 (...)* @test to i32 (i32, i32, i32, ...)*)(i32 %2, i32 3, i32 0) #3, !dbg !25
call void @llvm.dbg.value(metadata i16 5, metadata !13, metadata !DIExpression()), !dbg !18
%9 = add nsw i32 %7, 1, !dbg !27
call void @llvm.dbg.value(metadata i32 %9, metadata !11, metadata !DIExpression(DW_OP_LLVM_convert, 32, DW_ATE_unsigned, DW_OP_LLVM_convert, 8, DW_ATE_unsigned, DW_OP_stack_value)), !dbg !18
br label %1, !dbg !28, !llvm.loop !29
10: ; preds = %1
ret void, !dbg !33
}
declare dso_local i32 @test(...) local_unnamed_addr #1
; Function Attrs: nounwind uwtable
define dso_local i32 @main() local_unnamed_addr #0 !dbg !34 {
call void @a(), !dbg !37
ret i32 0, !dbg !38
}
[ . . .]
!13 = !DILocalVariable(name: "l_141", scope: !7, file: !1, line: 4, type: !14)
```
The second call to `llvm.dbg.value()` intrinsic has `undef` as first parameter when called with metadata `!13` (associated with `l_141`), thus making the variable not available during debugging.
</pre>
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