}
+/// \brief Check if the order of \p Op0 and \p Op1 as operand in an ICmpInst
+/// should be swapped.
+/// The descision is based on how many times these two operands are reused
+/// as subtract operands and their positions in those instructions.
+/// The rational is that several architectures use the same instruction for
+/// both subtract and cmp, thus it is better if the order of those operands
+/// match.
+/// \return true if Op0 and Op1 should be swapped.
+static bool swapMayExposeCSEOpportunities(const Value * Op0,
+ const Value * Op1) {
+ // Filter out pointer value as those cannot appears directly in subtract.
+ // FIXME: we may want to go through inttoptrs or bitcasts.
+ if (Op0->getType()->isPointerTy())
+ return false;
+ // Count every uses of both Op0 and Op1 in a subtract.
+ // Each time Op0 is the first operand, count -1: swapping is bad, the
+ // subtract has already the same layout as the compare.
+ // Each time Op0 is the second operand, count +1: swapping is good, the
+ // subtract has a diffrent layout as the compare.
+ // At the end, if the benefit is greater than 0, Op0 should come second to
+ // expose more CSE opportunities.
+ int GlobalSwapBenefits = 0;
+ for (Value::const_use_iterator UI = Op0->use_begin(), UIEnd = Op0->use_end(); UI != UIEnd; ++UI) {
+ const BinaryOperator *BinOp = dyn_cast<BinaryOperator>(*UI);
+ if (!BinOp || BinOp->getOpcode() != Instruction::Sub)
+ continue;
+ // If Op0 is the first argument, this is not beneficial to swap the
+ // arguments.
+ int LocalSwapBenefits = -1;
+ unsigned Op1Idx = 1;
+ if (BinOp->getOperand(Op1Idx) == Op0) {
+ Op1Idx = 0;
+ LocalSwapBenefits = 1;
+ }
+ if (BinOp->getOperand(Op1Idx) != Op1)
+ continue;
+ GlobalSwapBenefits += LocalSwapBenefits;
+ }
+ return GlobalSwapBenefits > 0;
+}
+
Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
bool Changed = false;
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
+ unsigned Op0Cplxity = getComplexity(Op0);
+ unsigned Op1Cplxity = getComplexity(Op1);
/// Orders the operands of the compare so that they are listed from most
/// complex to least complex. This puts constants before unary operators,
/// before binary operators.
- if (getComplexity(Op0) < getComplexity(Op1)) {
+ if (Op0Cplxity < Op1Cplxity ||
+ (Op0Cplxity == Op1Cplxity &&
+ swapMayExposeCSEOpportunities(Op0, Op1))) {
I.swapOperands();
std::swap(Op0, Op1);
Changed = true;
%cmp = icmp uge i32 %sub, 4
ret i1 %cmp
}
+
+; CHECK-LABEL: @icmp_swap_operands_for_cse
+; CHECK: [[CMP:%[a-z0-9]+]] = icmp ult i32 %X, %Y
+; CHECK-NEXT: br i1 [[CMP]], label %true, label %false
+; CHECK: ret i1
+define i1 @icmp_swap_operands_for_cse(i32 %X, i32 %Y) {
+entry:
+ %sub = sub i32 %X, %Y
+ %cmp = icmp ugt i32 %Y, %X
+ br i1 %cmp, label %true, label %false
+true:
+ %restrue = trunc i32 %sub to i1
+ br label %end
+false:
+ %shift = lshr i32 %sub, 4
+ %resfalse = trunc i32 %shift to i1
+ br label %end
+end:
+ %res = phi i1 [%restrue, %true], [%resfalse, %false]
+ ret i1 %res
+}
+
+; CHECK-LABEL: @icmp_swap_operands_for_cse2
+; CHECK: [[CMP:%[a-z0-9]+]] = icmp ult i32 %X, %Y
+; CHECK-NEXT: br i1 [[CMP]], label %true, label %false
+; CHECK: ret i1
+define i1 @icmp_swap_operands_for_cse2(i32 %X, i32 %Y) {
+entry:
+ %cmp = icmp ugt i32 %Y, %X
+ br i1 %cmp, label %true, label %false
+true:
+ %sub = sub i32 %X, %Y
+ %sub1 = sub i32 %X, %Y
+ %add = add i32 %sub, %sub1
+ %restrue = trunc i32 %add to i1
+ br label %end
+false:
+ %sub2 = sub i32 %Y, %X
+ %resfalse = trunc i32 %sub2 to i1
+ br label %end
+end:
+ %res = phi i1 [%restrue, %true], [%resfalse, %false]
+ ret i1 %res
+}
+
+; CHECK-LABEL: @icmp_do_not_swap_operands_for_cse
+; CHECK: [[CMP:%[a-z0-9]+]] = icmp ugt i32 %Y, %X
+; CHECK-NEXT: br i1 [[CMP]], label %true, label %false
+; CHECK: ret i1
+define i1 @icmp_do_not_swap_operands_for_cse(i32 %X, i32 %Y) {
+entry:
+ %cmp = icmp ugt i32 %Y, %X
+ br i1 %cmp, label %true, label %false
+true:
+ %sub = sub i32 %X, %Y
+ %restrue = trunc i32 %sub to i1
+ br label %end
+false:
+ %sub2 = sub i32 %Y, %X
+ %resfalse = trunc i32 %sub2 to i1
+ br label %end
+end:
+ %res = phi i1 [%restrue, %true], [%resfalse, %false]
+ ret i1 %res
+}