Speculatively execute a block when the the block is the then part of a triangle shape and it contains a single, side effect free, cheap instruction. The branch is eliminated by adding a select instruction. i.e.

Turn                                                                                                                                                                                                       
BB:                                                                                                                                                                                                        
    %t1 = icmp                                                                                                                                                                                             
    br i1 %t1, label %BB1, label %BB2                                                                                                                                                                      
BB1:                                                                                                                                                                                                       
    %t3 = add %t2, c                                                                                                                                                                                       
    br label BB2                                                                                                                                                                                           
BB2:                                                                                                                                                                                                       
=>                                                                                                                                                                                                         
BB:                                                                                                                                                                                                        
    %t1 = icmp                                                                                                                                                                                             
    %t4 = add %t2, c                                                                                                                                                                                       
    %t3 = select i1 %t1, %t2, %t3


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52073 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 21 insertions, 0 deletions

diff --git a/test/Transforms/SimplifyCFG/SpeculativeExec.ll b/test/Transforms/SimplifyCFG/SpeculativeExec.ll
new file mode 100644
index 0000000..2be9124
--- /dev/null
+++ b/test/Transforms/SimplifyCFG/SpeculativeExec.ll
@@ -0,0 +1,21 @@
+; RUN: llvm-as < %s | opt -simplifycfg  | llvm-dis | grep select
+; RUN: llvm-as < %s | opt -simplifycfg  | llvm-dis | grep br | count 2
+
+define i32 @t2(i32 %a, i32 %b, i32 %c) nounwind  {
+entry:
+        %tmp1 = icmp eq i32 %b, 0
+        br i1 %tmp1, label %bb1, label %bb3
+
+bb1:            ; preds = %entry
+	%tmp2 = icmp sgt i32 %c, 1
+	br i1 %tmp2, label %bb2, label %bb3
+
+bb2:		; preds = bb1
+	%tmp3 = add i32 %a, 1
+	br label %bb3
+
+bb3:		; preds = %bb2, %entry
+	%tmp4 = phi i32 [ %b, %entry ], [ %a, %bb1 ], [ %tmp3, %bb2 ]
+        %tmp5 = sub i32 %tmp4, 1
+	ret i32 %tmp5
+}