Refine the change in r52258 for avoiding use-before-def conditions

when changing the stride of a comparison so that it's slightly
more precise, by having it scan the instruction list to determine
if there is a use of the condition after the point where the
condition will be inserted.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52371 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 43 insertions, 12 deletions

diff --git a/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index e9976c5..f447292 100644
--- a/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -1530,15 +1530,6 @@ namespace {
 ICmpInst *LoopStrengthReduce::ChangeCompareStride(Loop *L, ICmpInst *Cond,
                                                 IVStrideUse* &CondUse,
                                                 const SCEVHandle* &CondStride) {
-  // Forgo this transformation if the condition has multiple uses. This is
-  // over-conservative, but simpler than alternatives. It guards against
-  // comparisons with a use that occurs earlier than the add instruction for the
-  // new stride index.  See
-  // test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness.ll
-  // for an example of this situation.
-  if (!Cond->hasOneUse())
-    return Cond;
-
   if (StrideOrder.size() < 2 ||
       IVUsesByStride[*CondStride].Users.size() != 1)
     return Cond;
@@ -1653,6 +1644,18 @@ ICmpInst *LoopStrengthReduce::ChangeCompareStride(Loop *L, ICmpInst *Cond,
     }
   }
 
+  // Forgo this transformation if it the increment happens to be
+  // unfortunately positioned after the condition, and the condition
+  // has multiple uses which prevent it from being moved immediately
+  // before the branch. See
+  // test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness-*.ll
+  // for an example of this situation.
+  if (!Cond->hasOneUse())
+    for (BasicBlock::iterator I = Cond, E = Cond->getParent()->end();
+         I != E; ++I)
+      if (I == NewIncV)
+        return Cond;
+
   if (NewCmpVal != CmpVal) {
     // Create a new compare instruction using new stride / iv.
     ICmpInst *OldCond = Cond;
diff --git a/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness-0.ll b/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness-0.ll
new file mode 100644
index 0000000..33b0522
--- /dev/null
+++ b/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness-0.ll
@@ -0,0 +1,24 @@
+; RUN: llvm-as < %s | llc --x86-asm-syntax=att | grep {cmpl	\$4}
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
+target triple = "x86_64-apple-darwin9"
+
+; This is like change-compare-stride-trickiness-1.ll except the comparison
+; happens before the relevant use, so the comparison stride can't be
+; easily changed.
+
+define void @foo() {
+entry:
+	br label %loop
+
+loop:
+	%indvar = phi i32 [ 0, %entry ], [ %i.2.0.us1534, %loop ]		; <i32> [#uses=1]
+	%i.2.0.us1534 = add i32 %indvar, 1		; <i32> [#uses=3]
+	%tmp611.us1535 = icmp eq i32 %i.2.0.us1534, 4		; <i1> [#uses=2]
+	%tmp623.us1538 = select i1 %tmp611.us1535, i32 6, i32 0		; <i32> [#uses=0]
+	%tmp628.us1540 = shl i32 %i.2.0.us1534, 1		; <i32> [#uses=1]
+	%tmp645646647.us1547 = sext i32 %tmp628.us1540 to i64		; <i64> [#uses=0]
+	br i1 %tmp611.us1535, label %exit, label %loop
+
+exit:
+	ret void
+}
diff --git a/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness.ll b/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness-1.ll
index 9a54b7b..06e2312 100644
--- a/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness.ll
+++ b/test/Transforms/LoopStrengthReduce/change-compare-stride-trickiness-1.ll
@@ -1,7 +1,11 @@
-; RUN: llvm-as < %s | llc
+; RUN: llvm-as < %s | llc --x86-asm-syntax=att | grep {cmpl	\$8}
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
 target triple = "x86_64-apple-darwin9"
 
+; This is like change-compare-stride-trickiness-0.ll except the comparison
+; happens after the relevant use, so the comparison stride can be
+; easily changed.
+
 define void @foo() {
 entry:
 	br label %loop
@@ -9,10 +13,10 @@ entry:
 loop:
 	%indvar = phi i32 [ 0, %entry ], [ %i.2.0.us1534, %loop ]		; <i32> [#uses=1]
 	%i.2.0.us1534 = add i32 %indvar, 1		; <i32> [#uses=3]
-	%tmp611.us1535 = icmp eq i32 %i.2.0.us1534, 4		; <i1> [#uses=2]
-	%tmp623.us1538 = select i1 %tmp611.us1535, i32 6, i32 0		; <i32> [#uses=0]
 	%tmp628.us1540 = shl i32 %i.2.0.us1534, 1		; <i32> [#uses=1]
 	%tmp645646647.us1547 = sext i32 %tmp628.us1540 to i64		; <i64> [#uses=0]
+	%tmp611.us1535 = icmp eq i32 %i.2.0.us1534, 4		; <i1> [#uses=2]
+	%tmp623.us1538 = select i1 %tmp611.us1535, i32 6, i32 0		; <i32> [#uses=0]
 	br i1 %tmp611.us1535, label %exit, label %loop
 
 exit: