[InstSimplify] Simplify saturating add/sub + icmp

If a saturating add/sub has one constant operand, then we can
determine the possible range of outputs it can produce, and simplify
an icmp comparison based on that.

The implementation is based on a similar existing mechanism for
simplifying binary operator + icmps.

Differential Revision: https://reviews.llvm.org/D55735

llvm-svn: 349369

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -2630,6 +2630,70 @@ static void setLimitsForBinOp(BinaryOperator &BO, APInt &Lower, APInt &Upper,
   }
 }
 
+/// Some intrinsics with a constant operand have an easy-to-compute range of
+/// outputs. This can be used to fold a comparison to always true or always
+/// false.
+static void setLimitsForIntrinsic(IntrinsicInst &II, APInt &Lower,
+                                  APInt &Upper) {
+  unsigned Width = Lower.getBitWidth();
+  const APInt *C;
+  switch (II.getIntrinsicID()) {
+  case Intrinsic::uadd_sat:
+    // uadd.sat(x, C) produces [C, UINT_MAX].
+    if (match(II.getOperand(0), m_APInt(C)) ||
+        match(II.getOperand(1), m_APInt(C)))
+      Lower = *C;
+    break;
+  case Intrinsic::sadd_sat:
+    if (match(II.getOperand(0), m_APInt(C)) ||
+        match(II.getOperand(1), m_APInt(C))) {
+      if (C->isNegative()) {
+        // sadd.sat(x, -C) produces [SINT_MIN, SINT_MAX + (-C)].
+        Lower = APInt::getSignedMinValue(Width);
+        Upper = APInt::getSignedMaxValue(Width) + *C + 1;
+      } else {
+        // sadd.sat(x, +C) produces [SINT_MIN + C, SINT_MAX].
+        Lower = APInt::getSignedMinValue(Width) + *C;
+        Upper = APInt::getSignedMaxValue(Width) + 1;
+      }
+    }
+    break;
+  case Intrinsic::usub_sat:
+    // usub.sat(C, x) produces [0, C].
+    if (match(II.getOperand(0), m_APInt(C)))
+      Upper = *C + 1;
+    // usub.sat(x, C) produces [0, UINT_MAX - C].
+    else if (match(II.getOperand(1), m_APInt(C)))
+      Upper = APInt::getMaxValue(Width) - *C + 1;
+    break;
+  case Intrinsic::ssub_sat:
+    if (match(II.getOperand(0), m_APInt(C))) {
+      if (C->isNegative()) {
+        // ssub.sat(-C, x) produces [SINT_MIN, -SINT_MIN + (-C)].
+        Lower = APInt::getSignedMinValue(Width);
+        Upper = *C - APInt::getSignedMinValue(Width) + 1;
+      } else {
+        // ssub.sat(+C, x) produces [-SINT_MAX + C, SINT_MAX].
+        Lower = *C - APInt::getSignedMaxValue(Width);
+        Upper = APInt::getSignedMaxValue(Width) + 1;
+      }
+    } else if (match(II.getOperand(1), m_APInt(C))) {
+      if (C->isNegative()) {
+        // ssub.sat(x, -C) produces [SINT_MIN - (-C), SINT_MAX]:
+        Lower = APInt::getSignedMinValue(Width) - *C;
+        Upper = APInt::getSignedMaxValue(Width) + 1;
+      } else {
+        // ssub.sat(x, +C) produces [SINT_MIN, SINT_MAX - C].
+        Lower = APInt::getSignedMinValue(Width);
+        Upper = APInt::getSignedMaxValue(Width) - *C + 1;
+      }
+    }
+    break;
+  default:
+    break;
+  }
+}
+
 static Value *simplifyICmpWithConstant(CmpInst::Predicate Pred, Value *LHS,
                                        Value *RHS, const InstrInfoQuery &IIQ) {
   Type *ITy = GetCompareTy(RHS); // The return type.
@@ -2663,6 +2727,8 @@ static Value *simplifyICmpWithConstant(CmpInst::Predicate Pred, Value *LHS,
   APInt Upper = APInt(Width, 0);
   if (auto *BO = dyn_cast<BinaryOperator>(LHS))
     setLimitsForBinOp(*BO, Lower, Upper, IIQ);
+  else if (auto *II = dyn_cast<IntrinsicInst>(LHS))
+    setLimitsForIntrinsic(*II, Lower, Upper);
 
   ConstantRange LHS_CR =
       Lower != Upper ? ConstantRange(Lower, Upper) : ConstantRange(Width, true);

llvm/test/Transforms/InstSimplify/saturating-add-sub.ll

@@ -408,9 +408,7 @@ define <2 x i8> @ssub_vector_same(<2 x i8> %a) {
 
 define i1 @uadd_icmp_op0_known(i8 %a) {
 ; CHECK-LABEL: @uadd_icmp_op0_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.uadd.sat.i8(i8 10, i8 [[A:%.*]])
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp uge i8 [[B]], 10
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.uadd.sat.i8(i8 10, i8 %a)
   %c = icmp uge i8 %b, 10
@@ -430,9 +428,7 @@ define i1 @uadd_icmp_op0_unknown(i8 %a) {
 
 define i1 @uadd_icmp_op1_known(i8 %a) {
 ; CHECK-LABEL: @uadd_icmp_op1_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.uadd.sat.i8(i8 [[A:%.*]], i8 10)
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp uge i8 [[B]], 10
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.uadd.sat.i8(i8 %a, i8 10)
   %c = icmp uge i8 %b, 10
@@ -452,9 +448,7 @@ define i1 @uadd_icmp_op1_unknown(i8 %a) {
 
 define i1 @sadd_icmp_op0_pos_known(i8 %a) {
 ; CHECK-LABEL: @sadd_icmp_op0_pos_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.sadd.sat.i8(i8 10, i8 [[A:%.*]])
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp sge i8 [[B]], -118
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.sadd.sat.i8(i8 10, i8 %a)
   %c = icmp sge i8 %b, -118
@@ -474,9 +468,7 @@ define i1 @sadd_icmp_op0_pos_unknown(i8 %a) {
 
 define i1 @sadd_icmp_op0_neg_known(i8 %a) {
 ; CHECK-LABEL: @sadd_icmp_op0_neg_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.sadd.sat.i8(i8 -10, i8 [[A:%.*]])
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp sle i8 [[B]], 117
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.sadd.sat.i8(i8 -10, i8 %a)
   %c = icmp sle i8 %b, 117
@@ -496,9 +488,7 @@ define i1 @sadd_icmp_op0_neg_unknown(i8 %a) {
 
 define i1 @sadd_icmp_op1_pos_known(i8 %a) {
 ; CHECK-LABEL: @sadd_icmp_op1_pos_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.sadd.sat.i8(i8 [[A:%.*]], i8 10)
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp sge i8 [[B]], -118
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.sadd.sat.i8(i8 %a, i8 10)
   %c = icmp sge i8 %b, -118
@@ -518,9 +508,7 @@ define i1 @sadd_icmp_op1_pos_unknown(i8 %a) {
 
 define i1 @sadd_icmp_op1_neg_known(i8 %a) {
 ; CHECK-LABEL: @sadd_icmp_op1_neg_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.sadd.sat.i8(i8 [[A:%.*]], i8 -10)
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp sle i8 [[B]], 117
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.sadd.sat.i8(i8 %a, i8 -10)
   %c = icmp sle i8 %b, 117
@@ -540,9 +528,7 @@ define i1 @sadd_icmp_op1_neg_unknown(i8 %a) {
 
 define i1 @usub_icmp_op0_known(i8 %a) {
 ; CHECK-LABEL: @usub_icmp_op0_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.usub.sat.i8(i8 10, i8 [[A:%.*]])
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp ule i8 [[B]], 10
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.usub.sat.i8(i8 10, i8 %a)
   %c = icmp ule i8 %b, 10
@@ -562,9 +548,7 @@ define i1 @usub_icmp_op0_unknown(i8 %a) {
 
 define i1 @usub_icmp_op1_known(i8 %a) {
 ; CHECK-LABEL: @usub_icmp_op1_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[A:%.*]], i8 10)
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp ule i8 [[B]], -11
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.usub.sat.i8(i8 %a, i8 10)
   %c = icmp ule i8 %b, 245
@@ -584,9 +568,7 @@ define i1 @usub_icmp_op1_unknown(i8 %a) {
 
 define i1 @ssub_icmp_op0_pos_known(i8 %a) {
 ; CHECK-LABEL: @ssub_icmp_op0_pos_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.ssub.sat.i8(i8 10, i8 [[A:%.*]])
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp sge i8 [[B]], -117
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.ssub.sat.i8(i8 10, i8 %a)
   %c = icmp sge i8 %b, -117
@@ -606,9 +588,7 @@ define i1 @ssub_icmp_op0_pos_unknown(i8 %a) {
 
 define i1 @ssub_icmp_op0_neg_known(i8 %a) {
 ; CHECK-LABEL: @ssub_icmp_op0_neg_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.ssub.sat.i8(i8 -10, i8 [[A:%.*]])
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp sle i8 [[B]], 118
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.ssub.sat.i8(i8 -10, i8 %a)
   %c = icmp sle i8 %b, 118
@@ -629,9 +609,7 @@ define i1 @ssub_icmp_op0_neg_unknown(i8 %a) {
 ; Peculiar case: ssub.sat(0, x) is never signed min.
 define i1 @ssub_icmp_op0_zero(i8 %a) {
 ; CHECK-LABEL: @ssub_icmp_op0_zero(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.ssub.sat.i8(i8 0, i8 [[A:%.*]])
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp ne i8 [[B]], -128
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.ssub.sat.i8(i8 0, i8 %a)
   %c = icmp ne i8 %b, -128
@@ -640,9 +618,7 @@ define i1 @ssub_icmp_op0_zero(i8 %a) {
 
 define i1 @ssub_icmp_op1_pos_known(i8 %a) {
 ; CHECK-LABEL: @ssub_icmp_op1_pos_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.ssub.sat.i8(i8 [[A:%.*]], i8 10)
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp sle i8 [[B]], 117
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.ssub.sat.i8(i8 %a, i8 10)
   %c = icmp sle i8 %b, 117
@@ -662,9 +638,7 @@ define i1 @ssub_icmp_op1_pos_unknown(i8 %a) {
 
 define i1 @ssub_icmp_op1_neg_known(i8 %a) {
 ; CHECK-LABEL: @ssub_icmp_op1_neg_known(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.ssub.sat.i8(i8 [[A:%.*]], i8 -10)
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp sge i8 [[B]], -118
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.ssub.sat.i8(i8 %a, i8 -10)
   %c = icmp sge i8 %b, -118
@@ -684,9 +658,7 @@ define i1 @ssub_icmp_op1_neg_unknown(i8 %a) {
 
 define i1 @ssub_icmp_op1_smin(i8 %a) {
 ; CHECK-LABEL: @ssub_icmp_op1_smin(
-; CHECK-NEXT:    [[B:%.*]] = call i8 @llvm.ssub.sat.i8(i8 [[A:%.*]], i8 -128)
+; CHECK-NEXT:    ret i1 true
-; CHECK-NEXT:    [[C:%.*]] = icmp sge i8 [[B]], 0
-; CHECK-NEXT:    ret i1 [[C]]
 ;
   %b = call i8 @llvm.ssub.sat.i8(i8 %a, i8 -128)
   %c = icmp sge i8 %b, 0