Add heuristics for irreducible loop metadata under PGO

Summary:
Add the following heuristics for irreducible loop metadata:

- When an irreducible loop header is missing the loop header weight metadata,
  give it the minimum weight seen among other headers.
- Annotate indirectbr targets with the loop header weight metadata (as they are
  likely to become irreducible loop headers after indirectbr tail duplication.)

These greatly improve the accuracy of the block frequency info of the Python
interpreter loop (eg. from ~3-16x off down to ~40-55% off) and the Python
performance (eg. unpack_sequence from ~50% slower to ~8% faster than GCC) due to
better register allocation under PGO.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39980

llvm-svn: 318693

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/test/Transforms/PGOProfile/irreducible.ll b/llvm/test/Transforms/PGOProfile/irreducible.ll
index 9b2c8f638ed..9394b724f7e 100644
--- a/llvm/test/Transforms/PGOProfile/irreducible.ll
+++ b/llvm/test/Transforms/PGOProfile/irreducible.ll
@@ -91,6 +91,7 @@ sw.bb:                                            ; preds = %indirectgoto, %disp
 
 TARGET_1:                                         ; preds = %indirectgoto
   br label %sw.bb6
+; USE: br label %sw.bb6, !irr_loop {{.*}}
 
 sw.bb6:                                           ; preds = %TARGET_1, %dispatch_op.sw.bb6_crit_edge
   br i1 %tobool, label %dispatch_op, label %if.then
@@ -102,6 +103,7 @@ if.then:                                          ; preds = %sw.bb6
 
 TARGET_2:                                         ; preds = %indirectgoto
   br label %sw.bb15
+; USE: br label %sw.bb15, !irr_loop {{.*}}
 
 sw.bb15:                                          ; preds = %TARGET_2, %dispatch_op
   br i1 %tobool, label %if.then18, label %exit