[DebugInfo][InstMerge] Fix -debugify for phi node created by -mldst-motion

Summary:
-mldst-motion creates a new phi node without any debug info. Use the merged debug location from the incoming stores to fix this.

Fixes PR38177. The test case here is (somewhat) simplified from:

```
struct S {
  int foo;
  void fn(int bar);
};
void S::fn(int bar) {
  if (bar)
    foo = 1;
  else
    foo = 0;
}
```

Reviewers: dblaikie, gbedwell, aprantl, vsk

Reviewed By: vsk

Subscribers: vsk, JDevlieghere, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D54019

llvm-svn: 346027

1 files changed, 32 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstMerge/st_sink_check_debug.ll b/llvm/test/Transforms/InstMerge/st_sink_check_debug.ll
new file mode 100644
index 00000000000..94d46a58f4c
--- /dev/null
+++ b/llvm/test/Transforms/InstMerge/st_sink_check_debug.ll
@@ -0,0 +1,32 @@
+; RUN: opt < %s -S -debugify -mldst-motion -o - | FileCheck %s
+
+%struct.S = type { i32 }
+
+define dso_local void @foo(%struct.S* %this, i32 %bar) {
+entry:
+  %this.addr = alloca %struct.S*, align 8
+  %bar.addr = alloca i32, align 4
+  store %struct.S* %this, %struct.S** %this.addr, align 8
+  store i32 %bar, i32* %bar.addr, align 4
+  %this1 = load %struct.S*, %struct.S** %this.addr, align 8
+  %0 = load i32, i32* %bar.addr, align 4
+  %tobool = icmp ne i32 %0, 0
+  br i1 %tobool, label %if.then, label %if.else
+
+if.then:                                          ; preds = %entry
+  %foo = getelementptr inbounds %struct.S, %struct.S* %this1, i32 0, i32 0
+  store i32 1, i32* %foo, align 4
+  br label %if.end
+
+if.else:                                          ; preds = %entry
+  %foo2 = getelementptr inbounds %struct.S, %struct.S* %this1, i32 0, i32 0
+  store i32 0, i32* %foo2, align 4
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  ret void
+}
+
+; CHECK:      @foo
+; CHECK:      if.end: ; preds = %if.else, %if.then
+; CHECK-NEXT:   %.sink = phi {{.*}} !dbg