[PATCH] D137707: Move "auto-init" instructions to the dominator of their users

[Hans Wennborg via Phabricator via llvm-commits]

hans added a comment.

In D137707#4259567 <https://reviews.llvm.org/D137707#4259567>, @ayzhao wrote:

> I now have a reproducible (but non-reduced) testcase: https://crbug.com/1431366#c5
>
> This looks like a miscompile; the return parameter is not being initialized if we don't take the branch.

Here's a small repro based on that:

  $ cat /tmp/a.cc
  struct S {
    unsigned long long x;  
  };
  
  S g();
  
  S f(int a) {
    S ret;
    if (a == 42)
      ret = g();
    return ret;
  }
  
  $ build/bin/clang.bad -target i686-linux-gnu -c -ftrivial-auto-var-init=pattern -O2 /tmp/a.cc -S -emit-llvm -o -
  [...]
  define dso_local void @_Z1fi(ptr noalias nocapture writeonly sret(%struct.S) align 4 %agg.result, i32 noundef %a) local_unnamed_addr #0 {
  entry:
    %ref.tmp = alloca %struct.S, align 8
    %cmp = icmp eq i32 %a, 42
    br i1 %cmp, label %if.then, label %if.end
  
  if.then:                                          ; preds = %entry
    store i64 -1, ptr %agg.result, align 4, !annotation !6        <------ This used to be in the %entry block.
    call void @llvm.lifetime.start.p0(i64 8, ptr nonnull %ref.tmp) #3
    call void @_Z1gv(ptr nonnull sret(%struct.S) align 4 %ref.tmp)
    %0 = load i64, ptr %ref.tmp, align 8, !tbaa !7
    store i64 %0, ptr %agg.result, align 4, !tbaa !7
    call void @llvm.lifetime.end.p0(i64 8, ptr nonnull %ref.tmp) #3
    br label %if.end
  
  if.end:                                           ; preds = %if.then, %entry
    ret void
  }

This patch moved the `store i64 -1, ptr %agg.result` instruction from the `%entry` block to `%if.then`, meaning the return value doesn't always get initialized.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D137707/new/

https://reviews.llvm.org/D137707