Split loop exiting edges more aggressively.

PHIElimination splits critical edges when it predicts it can resolve
interference and eliminate copies. It doesn't split the edge if the
interference wouldn't be resolved anyway because the phi-use register is
live in the critical edge anyway.

Teach PHIElimination to split loop exiting edges with interference, even
if it wouldn't resolve the interference. This removes the necessary
copies from the loop, which is still an improvement from injecting the
copies into the loop.

The test case demonstrates the improvement. Before:

LBB0_1:
  cmpb  $0, (%rdx)
  leaq  1(%rdx), %rdx
  movl  %esi, %eax
  je  LBB0_1

After:

LBB0_1:
  cmpb  $0, (%rdx)
  leaq  1(%rdx), %rdx
  je  LBB0_1

  movl  %esi, %eax

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160571 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 30 insertions, 0 deletions

diff --git a/test/CodeGen/X86/phielim-split.ll b/test/CodeGen/X86/phielim-split.ll
new file mode 100644
index 0000000000..aa477359d6
--- /dev/null
+++ b/test/CodeGen/X86/phielim-split.ll
@@ -0,0 +1,30 @@
+; RUN: llc < %s -verify-machineinstrs | FileCheck %s
+target triple = "x86_64-apple-macosx10.8.0"
+
+; The critical edge from for.cond to if.end2 should be split to avoid injecting
+; copies into the loop. The use of %b after the loop causes interference that
+; makes a copy necessary.
+; <rdar://problem/11561842>
+;
+; CHECK: split_loop_exit
+; CHECK: %for.cond
+; CHECK-NOT: mov
+; CHECK: je
+
+define i32 @split_loop_exit(i32 %a, i32 %b, i8* nocapture %p) nounwind uwtable readonly ssp {
+entry:
+  %cmp = icmp sgt i32 %a, 10
+  br i1 %cmp, label %for.cond, label %if.end2
+
+for.cond:                                         ; preds = %entry, %for.cond
+  %p.addr.0 = phi i8* [ %incdec.ptr, %for.cond ], [ %p, %entry ]
+  %incdec.ptr = getelementptr inbounds i8* %p.addr.0, i64 1
+  %0 = load i8* %p.addr.0, align 1
+  %tobool = icmp eq i8 %0, 0
+  br i1 %tobool, label %for.cond, label %if.end2
+
+if.end2:                                          ; preds = %for.cond, %entry
+  %r.0 = phi i32 [ %a, %entry ], [ %b, %for.cond ]
+  %add = add nsw i32 %r.0, %b
+  ret i32 %add
+}