[SimlifyCFG] Prevent passes from destroying canonical loop structure, especially for nested loops

    
When eliminating or merging almost empty basic blocks, the existence of non-trivial PHI nodes
is currently used to recognize potential loops of which the block is the header and keep the block.
However, the current algorithm fails if the loops' exit condition is evaluated only with volatile
values hence no PHI nodes in the header. Especially when such a loop is an outer loop of a nested
loop, the loop is collapsed into a single loop which prevent later optimizations from being
applied (e.g., transforming nested loops into simplified forms and loop vectorization).
    
The patch augments the existing PHI node-based check by adding a pre-test if the BB actually
belongs to a set of loop headers and not eliminating it if yes.

llvm-svn: 264697

2 files changed, 8 insertions, 8 deletions

diff --git a/llvm/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll b/llvm/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll
index d536da1e8b6..a215be9d487 100644
--- a/llvm/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll
+++ b/llvm/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll
@@ -16,23 +16,23 @@ for.body:                                         ; preds = %for.inc, %for.body.
   %cmp1 = icmp eq i32 %a, 12345
   br i1 %cmp1, label %if.then, label %if.else, !prof !0
 ; CHECK: %cmp1 = icmp eq i32 %a, 12345
-; CHECK-NEXT: br i1 %cmp1, label %if.then.us, label %if.else, !prof !0
+; CHECK-NEXT: br i1 %cmp1, label %for.body.us, label %for.body, !prof !0
 if.then:                                          ; preds = %for.body
-; CHECK: if.then.us:
+; CHECK: for.body.us:
 ; CHECK: add nsw i32 %{{.*}}, 123
 ; CHECK: %exitcond.us = icmp eq i32 %inc.us, %b
-; CHECK: br i1 %exitcond.us, label %for.cond.cleanup, label %if.then.us
+; CHECK: br i1 %exitcond.us, label %for.cond.cleanup, label %for.body.us
   %add = add nsw i32 %add.i, 123
   br label %for.inc
 
 if.else:                                          ; preds = %for.body
   %mul = mul nsw i32 %mul.i, %b
   br label %for.inc
-; CHECK: if.else:
+; CHECK: for.body:
 ; CHECK: %mul = mul nsw i32 %mul.i, %b
 ; CHECK: %inc = add nuw nsw i32 %inc.i, 1
 ; CHECK: %exitcond = icmp eq i32 %inc, %b
-; CHECK: br i1 %exitcond, label %for.cond.cleanup, label %if.else
+; CHECK: br i1 %exitcond, label %for.cond.cleanup, label %for.body
 for.inc:                                          ; preds = %if.then, %if.else
   %mul.p = phi i32 [ %b, %if.then ], [ %mul, %if.else ]
   %add.p = phi i32 [ %add, %if.then ], [ %a, %if.else ]
diff --git a/llvm/test/Transforms/LoopUnswitch/infinite-loop.ll b/llvm/test/Transforms/LoopUnswitch/infinite-loop.ll
index 3d1c895edec..0aef9092a1f 100644
--- a/llvm/test/Transforms/LoopUnswitch/infinite-loop.ll
+++ b/llvm/test/Transforms/LoopUnswitch/infinite-loop.ll
@@ -16,10 +16,10 @@
 ; CHECK-NEXT: br i1 %a, label %entry.split, label %abort0.split
 
 ; CHECK: entry.split:
-; CHECK-NEXT: br i1 %b, label %cond.end, label %abort1.split
+; CHECK-NEXT: br i1 %b, label %for.body, label %abort1.split
 
-; CHECK: cond.end:
-; CHECK-NEXT: br label %cond.end
+; CHECK: for.body:
+; CHECK-NEXT: br label %for.body
 
 ; CHECK: abort0.split:
 ; CHECK-NEXT: call void @end0() [[NOR_NUW:#[0-9]+]]