[PATCH] D54411: [Codegen] Merge tail blocks with no successors after block placement

[Jim Lin via Phabricator via llvm-commits]

Jim added inline comments.


================
Comment at: lib/CodeGen/BranchFolding.cpp:1074
+  if (!EnableTailMerge)
+    return MadeChange;
 
----------------
This change is it neccessary?


================
Comment at: test/CodeGen/X86/tail-opts.ll:439
+  %1 = icmp eq i32 undef, 0
+  br i1 %1, label %return, label %bb12
 
----------------
RKSimon wrote:
> dmgreen wrote:
> > Can you explain this change? The comment makes it look like it should be the same test as above.
> > 
> > But it somehow has different fall-through branches with the switches?
> icmp with undef will now constant fold in SelectionDAG - you're probably being affected by that
This testcase tests that two `tail_call_me` can't be merged into one for performance issue.
But bby and bbx are likely the same. This patch makes bby and bbx merge into one. 
And make `tail_call_me` only need one. 
So the two comparison of switch in bby and bbx should be changed difference.

This is my testcase to keep `tail_call_me` tail-duplicated.

```
define void @two_nosize(i32 %x, i32 %y, i32 %z) nounwind {
entry:
  %0 = icmp eq i32 %x, 0
  br i1 %0, label %bbx, label %bby

bby:
  switch i32 %y, label %bb7 [
    i32 0, label %return
  ]

bb7:
  store volatile i32 0, i32* @XYZ
  tail call void @tail_call_me()
  ret void

bbx:
  switch i32 %z, label %bb12 [
    i32 -1, label %return
  ]

bb12:
  store volatile i32 0, i32* @XYZ
  tail call void @tail_call_me()
  ret void

return:
  ret void
}

```
Let two comparison compare with 0 or -1. So that tail call `tail_call_me` can be put on the fall through edge.



CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D54411/new/

https://reviews.llvm.org/D54411