[llvm] [DebugInfo][SimpleLoopUnswitch] Fix missing debug location updates for new terminators (PR #98789)

[Stephen Tozer via llvm-commits]

================
@@ -0,0 +1,98 @@
+; RUN: opt -passes='loop(simple-loop-unswitch)' -S < %s | FileCheck %s
+; RUN: opt -passes='loop-mssa(simple-loop-unswitch)' -S < %s | FileCheck %s
+
+; Check that SimpleLoopUnswitch's unswitchTrivialBranch() and unswitchTrivialSwitch()
+; propagates debug locations to the new terminators replacing the old ones.
+
+define i32 @test1(ptr %var, i1 %cond1, i1 %cond2) !dbg !5 {
+; CHECK-LABEL: define i32 @test1(
+; CHECK:       loop_begin:
+; CHECK-NEXT:    br label %[[CONTINUE:.*]], !dbg [[DBG8:![0-9]+]]
+;
+entry:
+  br label %loop_begin, !dbg !8
+
+loop_begin:                                       ; preds = %do_something, %entry
+  br i1 %cond1, label %continue, label %loop_exit, !dbg !9
+
+continue:                                         ; preds = %loop_begin
+  %var_val = load i32, ptr %var, align 4, !dbg !10
+  br i1 %cond2, label %do_something, label %loop_exit, !dbg !11
+
+do_something:                                     ; preds = %continue
+  call void @some_func(), !dbg !12
+  br label %loop_begin, !dbg !13
+
+loop_exit:                                        ; preds = %continue, %loop_begin
+  ret i32 0, !dbg !14
+}
+
+define i32 @test7(i32 %cond1, i32 %x, i32 %y) !dbg !15 {
+; CHECK-LABEL: define i32 @test7(
+; CHECK-SAME: i32 [[COND1:%.*]], i32 [[X:%.*]], i32 [[Y:%.*]])
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    switch i32 [[COND1]], label %[[ENTRY_SPLIT:.*]] [
+; CHECK-NEXT:      i32 0, label %[[LOOP_EXIT:.*]]
+; CHECK-NEXT:      i32 1, label %[[LOOP_EXIT]]
+; CHECK-NEXT:    ], !dbg [[DBG16:![0-9]+]]
----------------
SLTozer wrote:

I guess it's a slightly tricky one - the transform here is:
```
// Before
    do {
        switch(cond) {
            case 0:
            case 1:
                goto exit_loop;
            default:
                some_func();
        }
    } while (true);
// After
    switch(cond) {
        case 0:
        case 1:
            goto exit_loop;
        default:
            do {
                some_func();
            } while (true);
    }
```
I think I agree, the "new" switch instruction should have the same debug loc of the old switch instruction; when we evaluate the condition, it should be on the `switch` line, not the `do` line.


https://github.com/llvm/llvm-project/pull/98789