[LICM] Don't create more than one copy of an instruction per loop exit block when sinking.

Fixes exponential compilation complexity in PR19835, caused by
LICM::sink not handling the following pattern well:

f = op g
e = op f, g
d = op e
c = op d, e
b = op c
a = op b, c

When an instruction with N uses is sunk, each of its operands gets N
new uses (all of them - phi nodes). In the example above, if a had 1
use, c would have 2, e would have 4, and g would have 8.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211673 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 29 insertions, 0 deletions

diff --git a/test/Transforms/LICM/extra-copies.ll b/test/Transforms/LICM/extra-copies.ll
new file mode 100644
index 0000000000..ef52f9f404
--- /dev/null
+++ b/test/Transforms/LICM/extra-copies.ll
@@ -0,0 +1,29 @@
+; RUN: opt < %s -licm -S | FileCheck %s
+; PR19835
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @f(i32 %x) {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %entry, %for.body
+  %storemerge4 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %mul = mul nsw i32 %x, %x
+  %add2 = add nsw i32 %mul, %x
+  %mul3 = add nsw i32 %add2, %mul
+  %inc = add nsw i32 %storemerge4, 1
+  %cmp = icmp slt i32 %inc, 100
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.body
+  %a9.0.lcssa = phi i32 [ %mul3, %for.body ]
+  ret i32 %a9.0.lcssa
+}
+
+; Test that there is exactly one copy of mul nsw i32 %x, %x in the exit block.
+; CHECK: define i32 @f(i32 [[X:%.*]])
+; CHECK: for.end:
+; CHECK-NOT: mul nsw i32 [[X]], [[X]]
+; CHECK: mul nsw i32 [[X]], [[X]]
+; CHECK-NOT: mul nsw i32 [[X]], [[X]]