[PATCH] D53390: [DebugInfo][Dexter] Unreachable line stepped onto after SimplifyCFG.

[Carlos Alberto Enciso via Phabricator via llvm-commits]

CarlosAlbertoEnciso added inline comments.


================
Comment at: lib/Transforms/Utils/SimplifyCFG.cpp:1379
+  Instruction *InsertPt = BIParent->size() > 1
+                              ? BIParent->getTerminator()->getPrevNode()
+                              : BIParent->getTerminator();
----------------
CarlosAlbertoEnciso wrote:
> vsk wrote:
> > CarlosAlbertoEnciso wrote:
> > > CarlosAlbertoEnciso wrote:
> > > > vsk wrote:
> > > > > A bit nitty-gritty, but: Can the instruction before the terminator be a debug info intrinsic?
> > > > > 
> > > > > If so, I think the line location you pick be different if you switch from -gline-tables-only versus -g.
> > > > Thanks for your review.
> > > > 
> > > > Changing the test case to:
> > > > 
> > > > ```
> > > > int main() {
> > > >   volatile int foo = 0;
> > > > 
> > > >   int beards = 0;
> > > >   bool cond = foo == 4;
> > > >   if (cond)
> > > >     beards = 8;
> > > >   else
> > > >     beards = 4;
> > > > 
> > > >   volatile bool tmp;
> > > >   volatile bool *face = &tmp;
> > > >   *face = cond;
> > > >  
> > > >   return beards;
> > > > }
> > > > ```
> > > > 
> > > > The IR will look like:
> > > > 
> > > > ```
> > > >   ...
> > > >   %foo.0. = load volatile i32, i32* %foo, align 4, !dbg !28, !tbaa !23
> > > >   %cmp = icmp eq i32 %foo.0., 4, !dbg !28
> > > >   %frombool = zext i1 %cmp to i8, !dbg !28
> > > >   call void @llvm.dbg.value(metadata i8 %frombool, metadata !16, metadata !DIExpression()), !dbg !28
> > > >   br i1 %cmp, label %if.then, label %if.else, !dbg !29
> > > > 
> > > > if.then:                                          ; preds = %entry
> > > >   call void @llvm.dbg.value(metadata i32 8, metadata !15, metadata !DIExpression()), !dbg !27
> > > >   br label %if.end, !dbg !30
> > > > 
> > > > if.else:                                          ; preds = %entry
> > > >   call void @llvm.dbg.value(metadata i32 4, metadata !15, metadata !DIExpression()), !dbg !27
> > > >   br label %if.end
> > > > 
> > > > if.end:                                           ; preds = %if.else, %if.then
> > > > ...
> > > > ```
> > > > 
> > > > The instruction before the terminator is a debug info intrinsic.
> > > > 
> > > > 
> > > > If so, I think the line location you pick be different if you switch from -gline-tables-only versus -g.
> > > 
> > > IR for '-g'
> > > 
> > > ```
> > >   ...
> > >   %cmp = icmp eq i32 %foo.0., 4, !dbg !26
> > >   %frombool = zext i1 %cmp to i8, !dbg !26
> > >   call void @llvm.dbg.value(metadata i8 %frombool, metadata !16, metadata !DIExpression()), !dbg !26
> > >   br i1 %cmp, label %if.then, label %if.else, !dbg !27
> > >   ...
> > >   !26 = !DILocation(line: 5, scope: !8)
> > > ```
> > > 
> > > IR for '-gline-tables-only'
> > > 
> > > ```
> > >   ...
> > >   %cmp = icmp eq i32 %foo.0., 4, !dbg !15
> > >   %frombool = zext i1 %cmp to i8, !dbg !15
> > >   br i1 %cmp, label %if.then, label %if.else, !dbg !16
> > >   ...
> > >   !15 = !DILocation(line: 5, scope: !8)
> > >   ...
> > > ```
> > > InsertPts
> > > ```
> > >   call void @llvm.dbg.value(metadata i8 %frombool, metadata !16, metadata !DIExpression()), !dbg !26
> > > 
> > > &&
> > > 
> > >   %frombool = zext i1 %cmp to i8, !dbg !15
> > > ```
> > > 
> > > For both options, the line location of the insertion point is the same, debug location `!DILocation(line: 5, scope: !8)`.
> > > 
> > > 
> > I think it's still possible to construct cases where -g vs. -gline-tables-only produces different locations. What if you add `int bar = 0` on the line after `bool cond = foo == 4`? I see this:
> > 
> > ```
> >   %6 = zext i1 %5 to i8, !dbg !31
> >   call void @llvm.dbg.value(metadata i8 %6, metadata !15, metadata !DIExpression()), !dbg !31
> >   call void @llvm.dbg.value(metadata i32 0, metadata !17, metadata !DIExpression()), !dbg !32
> >   br i1 %5, label %7, label %8, !dbg !33
> > ```
> > 
> > Where !32 and !31 are on different lines. Link in godbolt: https://godbolt.org/z/VswzLx
> You are correct. By adding that line, the insertion points have different debug locations:
> 
> ```
>   call void @llvm.dbg.value(metadata i32 0, metadata !18, metadata !DIExpression()), !dbg !28
> !28 = !DILocation(line: 6, scope: !8)
> ```
> vs
> 
> ```
>   %frombool = zext i1 %cmp to i8, !dbg !15
> !15 = !DILocation(line: 5, scope: !8)
> ```
> 
> 
These are the IRs for both options:

```
  ...
  %cmp = icmp eq i32 %foo.0., 4, !dbg !27
  %frombool = zext i1 %cmp to i8, !dbg !27
  call void @llvm.dbg.value(metadata i8 %frombool, metadata !16, metadata !DIExpression()), !dbg !27
  call void @llvm.dbg.value(metadata i32 0, metadata !18, metadata !DIExpression()), !dbg !28
  br i1 %cmp, label %if.then, label %if.else, !dbg !29
  ...
!27 = !DILocation(line: 5, scope: !8)
!28 = !DILocation(line: 6, scope: !8)
```
&
```
  ...
  %cmp = icmp eq i32 %foo.0., 4, !dbg !15
  %frombool = zext i1 %cmp to i8, !dbg !15
  br i1 %cmp, label %if.then, label %if.else, !dbg !16
  ...
!15 = !DILocation(line: 5, scope: !8)
```

What I can propose to have the same behavior for both options (-g vs -gline-tables-only), is for the case when the insertion point is a debug intrinsic, to find the previous instruction that is not a debug intrinsic (within the same BB), to be used as insertion point.

Create a function `Instruction::getPrevNonDebugInstruction()` to get that instruction. We have already `Instruction::getNextNonDebugInstruction()`.

Doing that, then for the above IRs, for both cases, the insertion point will be:

```
  %frombool = zext i1 %cmp to i8, !dbg !27
```
or

```
%frombool = zext i1 %cmp to i8, !dbg !15
```

that have the same debug location.

What to do if the BB contains only debug instructions?



https://reviews.llvm.org/D53390