+/// FoldICmpAddOpCst - Fold "icmp pred (X+CI), X".
+Instruction *InstCombiner::FoldICmpAddOpCst(ICmpInst &ICI,
+ Value *X, ConstantInt *CI,
+ ICmpInst::Predicate Pred) {
+ // If we have X+0, exit early (simplifying logic below) and let it get folded
+ // elsewhere. icmp X+0, X -> icmp X, X
+ if (CI->isZero()) {
+ bool isTrue = ICmpInst::isTrueWhenEqual(Pred);
+ return ReplaceInstUsesWith(ICI, ConstantInt::get(ICI.getType(), isTrue));
+ }
+
+ // (X+4) == X -> false.
+ if (Pred == ICmpInst::ICMP_EQ)
+ return ReplaceInstUsesWith(ICI, ConstantInt::getFalse(X->getContext()));
+
+ // (X+4) != X -> true.
+ if (Pred == ICmpInst::ICMP_NE)
+ return ReplaceInstUsesWith(ICI, ConstantInt::getTrue(X->getContext()));
+
+ // From this point on, we know that (X+C <= X) --> (X+C < X) because C != 0,
+ // so the values can never be equal. Similiarly for all other "or equals"
+ // operators.
+
+ // (X+1) <u X --> X >u (MAXUINT-1) --> X != 255
+ // (X+2) <u X --> X >u (MAXUINT-2) --> X > 253
+ // (X+MAXUINT) <u X --> X >u (MAXUINT-MAXUINT) --> X != 0
+ if (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_ULE) {
+ Value *R = ConstantExpr::getSub(ConstantInt::get(CI->getType(), -1ULL), CI);
+ return new ICmpInst(ICmpInst::ICMP_UGT, X, R);
+ }
+
+ // (X+1) >u X --> X <u (0-1) --> X != 255
+ // (X+2) >u X --> X <u (0-2) --> X <u 254
+ // (X+MAXUINT) >u X --> X <u (0-MAXUINT) --> X <u 1 --> X == 0
+ if (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_UGE)
+ return new ICmpInst(ICmpInst::ICMP_ULT, X, ConstantExpr::getNeg(CI));
+
+ unsigned BitWidth = CI->getType()->getPrimitiveSizeInBits();
+ ConstantInt *SMax = ConstantInt::get(X->getContext(),
+ APInt::getSignedMaxValue(BitWidth));
+
+ // (X+ 1) <s X --> X >s (MAXSINT-1) --> X == 127
+ // (X+ 2) <s X --> X >s (MAXSINT-2) --> X >s 125
+ // (X+MAXSINT) <s X --> X >s (MAXSINT-MAXSINT) --> X >s 0
+ // (X+MINSINT) <s X --> X >s (MAXSINT-MINSINT) --> X >s -1
+ // (X+ -2) <s X --> X >s (MAXSINT- -2) --> X >s 126
+ // (X+ -1) <s X --> X >s (MAXSINT- -1) --> X != 127
+ if (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_SLE)
+ return new ICmpInst(ICmpInst::ICMP_SGT, X, ConstantExpr::getSub(SMax, CI));
+
+ // (X+ 1) >s X --> X <s (MAXSINT-(1-1)) --> X != 127
+ // (X+ 2) >s X --> X <s (MAXSINT-(2-1)) --> X <s 126
+ // (X+MAXSINT) >s X --> X <s (MAXSINT-(MAXSINT-1)) --> X <s 1
+ // (X+MINSINT) >s X --> X <s (MAXSINT-(MINSINT-1)) --> X <s -2
+ // (X+ -2) >s X --> X <s (MAXSINT-(-2-1)) --> X <s -126
+ // (X+ -1) >s X --> X <s (MAXSINT-(-1-1)) --> X == -128
+ assert(Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SGE);
+ Constant *C = ConstantInt::get(X->getContext(), CI->getValue()-1);
+ return new ICmpInst(ICmpInst::ICMP_SLT, X, ConstantExpr::getSub(SMax, C));
+}