[Loop Unswitch] Patch to selective unswitch only the reachable branch instructions.

Summary:
The iterative algorithm for Loop Unswitching may render some of the branches unreachable in the unswitched loops.
Given the exponential nature of the algorithm, this is quite an overhead.
This patch fixes this problem by selectively unswitching only those branches within a loop that are reachable from the loop header.

Reviewers: Michael Zolothukin, Anna Thomas, Weiming Zhao.
Subscribers: llvm-commits.

Differential Revision: http://reviews.llvm.org/D26299

llvm-svn: 287925

1 files changed, 62 insertions, 0 deletions

diff --git a/llvm/test/Transforms/LoopUnswitch/elseif-non-exponential-behavior.ll b/llvm/test/Transforms/LoopUnswitch/elseif-non-exponential-behavior.ll
new file mode 100644
index 00000000000..81897d872cd
--- /dev/null
+++ b/llvm/test/Transforms/LoopUnswitch/elseif-non-exponential-behavior.ll
@@ -0,0 +1,62 @@
+; RUN: opt -loop-unswitch -S - < %s | FileCheck %s
+
+;CHECK-LABEL: @b
+;CHECK: [[Loop1:for\.end.*]]:                              ; preds = %for.cond.us
+;CHECK-NEXT:  %[[PhiVar1:pdt.*]] = phi i32 [ %pdt.0.us, %for.cond.us ]
+;CHECK: [[Loop2:for\.end.*]]:                     ; preds = %for.cond.us1
+;CHECK-NEXT:  %[[PhiVar2:pdt.*]] = phi i32 [ %pdt.0.us2, %for.cond.us1 ]
+;CHECK: [[Loop3:for\.end.*]]:                        ; preds = %for.cond
+;CHECK-NEXT:  %[[PhiVar3:pdt.*]] = phi i32 [ %pdt.0, %for.cond ]
+;CHECK: [[Join1:for\.end.*]]:                                 ; preds = %[[Loop2]], %[[Loop3]]
+;CHECK-NEXT:  %[[PhiRes1:pdt.*]] = phi i32 [ %[[PhiVar3]], %[[Loop3]] ], [ %[[PhiVar2]], %[[Loop2]] ]
+;CHECK: for.end:                                          ; preds = %[[Loop1]], %[[Join1]]
+;CHECK-NEXT:  %[[PhiRes2:pdt.*]] = phi i32 [ %[[PhiRes1]], %[[Join1]] ], [ %[[PhiVar1]], %[[Loop1]] ]
+;CHECK-NEXT:  ret i32 %[[PhiRes2]]
+
+; Function Attrs: nounwind uwtable
+define i32 @b(i32 %x, i32 %y) #0 {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %pdt.0 = phi i32 [ 1, %entry ], [ %pdt.2, %for.inc ]
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 100
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %tobool = icmp ne i32 %x, 0
+  br i1 %tobool, label %if.then, label %if.else
+
+if.then:                                          ; preds = %for.body
+  %mul = mul nsw i32 %pdt.0, 2
+  br label %if.end6
+
+if.else:                                          ; preds = %for.body
+  %tobool1 = icmp ne i32 %y, 0
+  br i1 %tobool1, label %if.then2, label %if.else4
+
+if.then2:                                         ; preds = %if.else
+  %mul3 = mul nsw i32 %pdt.0, 3
+  br label %if.end
+
+if.else4:                                         ; preds = %if.else
+  %mul5 = mul nsw i32 %pdt.0, 4
+  br label %if.end
+
+if.end:                                           ; preds = %if.else4, %if.then2
+  %pdt.1 = phi i32 [ %mul3, %if.then2 ], [ %mul5, %if.else4 ]
+  br label %if.end6
+
+if.end6:                                          ; preds = %if.end, %if.then
+  %pdt.2 = phi i32 [ %mul, %if.then ], [ %pdt.1, %if.end ]
+  br label %for.inc
+
+for.inc:                                          ; preds = %if.end6
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret i32 %pdt.0
+}
+