Use a sign-extend instead of a zero-extend when promoting a

trip count value when the original loop iteration condition is
signed and the canonical induction variable won't undergo signed
overflow. This isn't required for correctness; it just preserves
more information about original loop iteration values.

Add a getTruncateOrSignExtend method to ScalarEvolution,
following getTruncateOrZeroExtend.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64918 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ScalarEvolution.h

@@ -264,6 +264,11 @@ namespace llvm {
     /// extended, it is zero extended.
     SCEVHandle getTruncateOrZeroExtend(const SCEVHandle &V, const Type *Ty);
 
+    /// getTruncateOrSignExtend - Return a SCEV corresponding to a conversion
+    /// of the input value to the specified type.  If the type must be
+    /// extended, it is sign extended.
+    SCEVHandle getTruncateOrSignExtend(const SCEVHandle &V, const Type *Ty);
+
     /// getIntegerSCEV - Given an integer or FP type, create a constant for the
     /// specified signed integer value and return a SCEV for the constant.
     SCEVHandle getIntegerSCEV(int Val, const Type *Ty);

lib/Analysis/ScalarEvolution.cpp

@@ -755,6 +755,21 @@ SCEVHandle ScalarEvolution::getTruncateOrZeroExtend(const SCEVHandle &V,
   return getZeroExtendExpr(V, Ty);
 }
 
+/// getTruncateOrSignExtend - Return a SCEV corresponding to a conversion
+/// of the input value to the specified type.  If the type must be
+/// extended, it is sign extended.
+SCEVHandle ScalarEvolution::getTruncateOrSignExtend(const SCEVHandle &V,
+                                                    const Type *Ty) {
+  const Type *SrcTy = V->getType();
+  assert(SrcTy->isInteger() && Ty->isInteger() &&
+         "Cannot truncate or sign extend with non-integer arguments!");
+  if (SrcTy->getPrimitiveSizeInBits() == Ty->getPrimitiveSizeInBits())
+    return V;  // No conversion
+  if (SrcTy->getPrimitiveSizeInBits() > Ty->getPrimitiveSizeInBits())
+    return getTruncateExpr(V, Ty);
+  return getSignExtendExpr(V, Ty);
+}
+
 // get - Get a canonical add expression, or something simpler if possible.
 SCEVHandle ScalarEvolution::getAddExpr(std::vector<SCEVHandle> &Ops) {
   assert(!Ops.empty() && "Cannot get empty add!");

lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -96,7 +96,8 @@ namespace {
                                    Value *IndVar,
                                    BasicBlock *ExitingBlock,
                                    BranchInst *BI,
-                                   SCEVExpander &Rewriter);
+                                   SCEVExpander &Rewriter,
+                                   bool SignExtendTripCount);
     void RewriteLoopExitValues(Loop *L, SCEV *IterationCount);
 
     void DeleteTriviallyDeadInstructions(SmallPtrSet<Instruction*, 16> &Insts);
@@ -235,7 +236,8 @@ void IndVarSimplify::LinearFunctionTestReplace(Loop *L,
                                    Value *IndVar,
                                    BasicBlock *ExitingBlock,
                                    BranchInst *BI,
-                                   SCEVExpander &Rewriter) {
+                                   SCEVExpander &Rewriter,
+                                   bool SignExtendTripCount) {
   // If the exiting block is not the same as the backedge block, we must compare
   // against the preincremented value, otherwise we prefer to compare against
   // the post-incremented value.
@@ -253,9 +255,16 @@ void IndVarSimplify::LinearFunctionTestReplace(Loop *L,
     if ((isa<SCEVConstant>(N) && !N->isZero()) ||
         SE->isLoopGuardedByCond(L, ICmpInst::ICMP_NE, N, Zero)) {
       // No overflow. Cast the sum.
+      if (SignExtendTripCount)
+        IterationCount = SE->getTruncateOrSignExtend(N, IndVar->getType());
+      else
         IterationCount = SE->getTruncateOrZeroExtend(N, IndVar->getType());
     } else {
       // Potential overflow. Cast before doing the add.
+      if (SignExtendTripCount)
+        IterationCount = SE->getTruncateOrSignExtend(IterationCount,
+                                                     IndVar->getType());
+      else
         IterationCount = SE->getTruncateOrZeroExtend(IterationCount,
                                                      IndVar->getType());
       IterationCount =
@@ -269,6 +278,10 @@ void IndVarSimplify::LinearFunctionTestReplace(Loop *L,
     CmpIndVar = L->getCanonicalInductionVariableIncrement();
   } else {
     // We have to use the preincremented value...
+    if (SignExtendTripCount)
+      IterationCount = SE->getTruncateOrSignExtend(IterationCount,
+                                                   IndVar->getType());
+    else
       IterationCount = SE->getTruncateOrZeroExtend(IterationCount,
                                                    IndVar->getType());
     CmpIndVar = IndVar;
@@ -464,10 +477,13 @@ static const Type *getEffectiveIndvarType(const PHINode *Phi) {
 
 /// TestOrigIVForWrap - Analyze the original induction variable
 /// that controls the loop's iteration to determine whether it
-/// would ever undergo signed or unsigned overflow.
+/// would ever undergo signed or unsigned overflow. Also, check
+/// whether an induction variable in the same type that starts
+/// at 0 would undergo signed overflow.
 ///
-/// In addition to setting the NoSignedWrap and NoUnsignedWrap
+/// In addition to setting the NoSignedWrap, NoUnsignedWrap, and
-/// variables, return the PHI for this induction variable.
+/// SignExtendTripCount variables, return the PHI for this induction
+/// variable.
 ///
 /// TODO: This duplicates a fair amount of ScalarEvolution logic.
 /// Perhaps this can be merged with ScalarEvolution::getIterationCount
@@ -477,7 +493,8 @@ static const PHINode *TestOrigIVForWrap(const Loop *L,
                                         const BranchInst *BI,
                                         const Instruction *OrigCond,
                                         bool &NoSignedWrap,
-                                        bool &NoUnsignedWrap) {
+                                        bool &NoUnsignedWrap,
+                                        bool &SignExtendTripCount) {
   // Verify that the loop is sane and find the exit condition.
   const ICmpInst *Cmp = dyn_cast<ICmpInst>(OrigCond);
   if (!Cmp) return 0;
@@ -590,9 +607,13 @@ static const PHINode *TestOrigIVForWrap(const Loop *L,
   // The original induction variable will start at some non-max value,
   // it counts up by one, and the loop iterates only while it remans
   // less than some value in the same type. As such, it will never wrap.
-  if (isSigned && !InitialVal->getValue().isMaxSignedValue())
+  if (isSigned && !InitialVal->getValue().isMaxSignedValue()) {
     NoSignedWrap = true;
-  else if (!isSigned && !InitialVal->getValue().isMaxValue())
+    // If the original induction variable starts at zero or greater,
+    // the trip count can be considered signed.
+    if (InitialVal->getValue().isNonNegative())
+      SignExtendTripCount = true;
+  } else if (!isSigned && !InitialVal->getValue().isMaxValue())
     NoUnsignedWrap = true;
   return PN;
 }
@@ -678,6 +699,7 @@ bool IndVarSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
   // using it.  We can currently only handle loops with a single exit.
   bool NoSignedWrap = false;
   bool NoUnsignedWrap = false;
+  bool SignExtendTripCount = false;
   const PHINode *OrigControllingPHI = 0;
   if (!isa<SCEVCouldNotCompute>(IterationCount) && ExitingBlock)
     // Can't rewrite non-branch yet.
@@ -686,14 +708,16 @@ bool IndVarSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
         // Determine if the OrigIV will ever undergo overflow.
         OrigControllingPHI =
           TestOrigIVForWrap(L, BI, OrigCond,
-                            NoSignedWrap, NoUnsignedWrap);
+                            NoSignedWrap, NoUnsignedWrap,
+                            SignExtendTripCount);
 
         // We'll be replacing the original condition, so it'll be dead.
         DeadInsts.insert(OrigCond);
       }
 
       LinearFunctionTestReplace(L, IterationCount, IndVar,
-                                ExitingBlock, BI, Rewriter);
+                                ExitingBlock, BI, Rewriter,
+                                SignExtendTripCount);
     }
 
   // Now that we have a canonical induction variable, we can rewrite any

test/Transforms/IndVarSimplify/preserve-signed-wrap.ll

@@ -1,5 +1,6 @@
 ; RUN: llvm-as < %s | opt -indvars | llvm-dis > %t
-; RUN: grep sext %t | count 1
+; RUN: grep sext %t | count 2
+; RUN: grep { = sext i32 %n to i64} %t
 ; RUN: grep phi %t | count 1
 ; RUN: grep {phi i64} %t
 

test/Transforms/IndVarSimplify/promote-iv-to-eliminate-casts.ll

@@ -1,6 +1,7 @@
-; RUN: llvm-as < %s | opt -indvars | llvm-dis | not grep sext
+; RUN: llvm-as < %s | opt -indvars | llvm-dis > %t
-
+; RUN: grep sext %t | count 2
-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"
+; RUN: grep { = sext i16 %N to i64} %t
+; RUN: grep { = sext i32 %count to i64} %t
 
 define i64 @test(i64* nocapture %first, i32 %count) nounwind readonly {
 entry:

test/Transforms/IndVarSimplify/signed-trip-count.ll

@@ -0,0 +1,31 @@
+; RUN: llvm-as < %s | opt -indvars | llvm-dis > %t
+; RUN: grep { = sext i32 %n} %t
+; RUN: grep phi %t | count 1
+; RUN: not grep zext %t
+
+define void @foo(i64* nocapture %x, i32 %n) nounwind {
+entry:
+	%tmp102 = icmp sgt i32 %n, 0		; <i1> [#uses=1]
+	br i1 %tmp102, label %bb.nph, label %return
+
+bb.nph:		; preds = %entry
+	br label %bb
+
+bb:		; preds = %bb7, %bb.nph
+	%i.01 = phi i32 [ %tmp6, %bb7 ], [ 0, %bb.nph ]		; <i32> [#uses=3]
+	%tmp1 = sext i32 %i.01 to i64		; <i64> [#uses=1]
+	%tmp4 = getelementptr i64* %x, i32 %i.01		; <i64*> [#uses=1]
+	store i64 %tmp1, i64* %tmp4, align 8
+	%tmp6 = add i32 %i.01, 1		; <i32> [#uses=2]
+	br label %bb7
+
+bb7:		; preds = %bb
+	%tmp10 = icmp slt i32 %tmp6, %n		; <i1> [#uses=1]
+	br i1 %tmp10, label %bb, label %bb7.return_crit_edge
+
+bb7.return_crit_edge:		; preds = %bb7
+	br label %return
+
+return:		; preds = %bb7.return_crit_edge, %entry
+	ret void
+}