[PATCH] D57149: [StackProtector] Skip analysing dead users in HasAddressTaken, PR40436

[Mikael Holmén via Phabricator via llvm-commits]

uabelho added a comment.

In D57149#1369777 <https://reviews.llvm.org/D57149#1369777>, @rnk wrote:

> An alternative way to handle this would be to remove unreachable code. I'm surprised unreachable code reaches this pass in a normal pipeline. I thought CGP removed it in normal operation, even at O0. However, removing unreachable code might invalidate the domtree (unless our utility for that updates it?).


An alternative that you would prefer?

I have to say that I think it makes sense that StackProtector can deal with the code on it's own since we never know when some dead code might reach StackProtector anyway, but perhaps there are strong reasons CGP really shouldn't leave dead code around?

I don't know CGP, but from the logs I see that it's after CGP that the dead blocks really get disconnected.
Before CGP:

  *** IR Dump Before CodeGen Prepare ***
  ; Function Attrs: sspreq
  define void @f1() #0 {
  entry:
    %d = alloca i16, align 1
    br label %for.cond
  
  for.cond:                                         ; preds = %for.cond, %entry
    br i1 true, label %for.cond, label %for.end
  
  for.end:                                          ; preds = %for.cond
    %tobool = icmp ne i16 undef, 0
    br label %for.cond1
  
  for.cond1:                                        ; preds = %for.body2, %for.end
    %d2 = phi i16* [ %spec.select, %for.body2 ], [ undef, %for.end ]
    br label %for.body2
  
  for.body2:                                        ; preds = %for.cond1
    %spec.select = select i1 undef, i16* %d, i16* %d2
    br label %for.cond1
  }

And then after:

  *** IR Dump After CodeGen Prepare ***
  ; Function Attrs: sspreq
  define void @f1() #0 {
  entry:
    %d = alloca i16, align 1
    br label %for.cond
  
  for.cond:                                         ; preds = %for.cond, %entry
    br label %for.cond
  
  for.body2:                                        ; preds = %for.body2
    %spec.select = select i1 undef, i16* %d, i16* %spec.select
    br label %for.body2
  }


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D57149/new/

https://reviews.llvm.org/D57149