Changes in directory llvm/lib/Transforms/Scalar:
LoopStrengthReduce.cpp updated: 1.137 -> 1.138
---
Log message:
Handle negative strides much more optimally. This compiles
X86/lsr-negative-stride.ll
into:
_t:
movl 8(%esp), %ecx
movl 4(%esp), %eax
cmpl %ecx, %eax
je LBB1_3 #bb17
LBB1_1: #bb
cmpl %ecx, %eax
jg LBB1_4 #cond_true
LBB1_2: #cond_false
subl %eax, %ecx
cmpl %ecx, %eax
jne LBB1_1 #bb
LBB1_3: #bb17
ret
LBB1_4: #cond_true
subl %ecx, %eax
cmpl %ecx, %eax
jne LBB1_1 #bb
jmp LBB1_3 #bb17
instead of:
_t:
subl $4, %esp
movl %esi, (%esp)
movl 12(%esp), %ecx
movl 8(%esp), %eax
cmpl %ecx, %eax
je LBB1_4 #bb17
LBB1_1: #bb.outer
movl %ecx, %edx
negl %edx
LBB1_2: #bb
cmpl %ecx, %eax
jle LBB1_5 #cond_false
LBB1_3: #cond_true
addl %edx, %eax
cmpl %ecx, %eax
jne LBB1_2 #bb
LBB1_4: #bb17
movl (%esp), %esi
addl $4, %esp
ret
LBB1_5: #cond_false
movl %ecx, %edx
subl %eax, %edx
movl %eax, %esi
addl %esi, %esi
cmpl %ecx, %esi
je LBB1_4 #bb17
LBB1_6: #cond_false.bb.outer_crit_edge
movl %edx, %ecx
jmp LBB1_1 #bb.outer
---
Diffs of the changes: (+26 -3)
LoopStrengthReduce.cpp | 29 ++++++++++++++++++++++++++---
1 files changed, 26 insertions(+), 3 deletions(-)
Index: llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
diff -u llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp:1.137
llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp:1.138
--- llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp:1.137 Fri May 11
17:40:34 2007
+++ llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp Fri May 18 20:22:21 2007
@@ -987,6 +987,20 @@
return Val.isUseOfPostIncrementedValue;
}
+/// isNonConstantNegative - REturn true if the specified scev is negated, but
+/// not a constant.
+static bool isNonConstantNegative(const SCEVHandle &Expr) {
+ SCEVMulExpr *Mul = dyn_cast<SCEVMulExpr>(Expr);
+ if (!Mul) return false;
+
+ // If there is a constant factor, it will be first.
+ SCEVConstant *SC = dyn_cast<SCEVConstant>(Mul->getOperand(0));
+ if (!SC) return false;
+
+ // Return true if the value is negative, this matches things like (-42 * V).
+ return SC->getValue()->getValue().isNegative();
+}
+
/// StrengthReduceStridedIVUsers - Strength reduce all of the users of a single
/// stride of IV. All of the users may have different starting values, and
this
/// may not be the only stride (we know it is if isOnlyStride is true).
@@ -1104,15 +1118,24 @@
// Add common base to the new Phi node.
NewPHI->addIncoming(CommonBaseV, Preheader);
+ // If the stride is negative, insert a sub instead of an add for the
+ // increment.
+ bool isNegative = isNonConstantNegative(Stride);
+ SCEVHandle IncAmount = Stride;
+ if (isNegative)
+ IncAmount = SCEV::getNegativeSCEV(Stride);
+
// Insert the stride into the preheader.
- Value *StrideV = PreheaderRewriter.expandCodeFor(Stride, PreInsertPt,
+ Value *StrideV = PreheaderRewriter.expandCodeFor(IncAmount, PreInsertPt,
ReplacedTy);
if (!isa<ConstantInt>(StrideV)) ++NumVariable;
// Emit the increment of the base value before the terminator of the loop
// latch block, and add it to the Phi node.
- SCEVHandle IncExp = SCEVAddExpr::get(SCEVUnknown::get(NewPHI),
- SCEVUnknown::get(StrideV));
+ SCEVHandle IncExp = SCEVUnknown::get(StrideV);
+ if (isNegative)
+ IncExp = SCEV::getNegativeSCEV(IncExp);
+ IncExp = SCEVAddExpr::get(SCEVUnknown::get(NewPHI), IncExp);
IncV = Rewriter.expandCodeFor(IncExp, LatchBlock->getTerminator(),
ReplacedTy);
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