[ValueTracking] Add subtraction support for setLimitsForBinOp (llvm#143618)

We can determine the range from a subtraction if it has nsw or nuw.

https://alive2.llvm.org/ce/z/tXAKVV

Author: AZero13

llvm/lib/Analysis/ValueTracking.cpp

@@ -9580,15 +9580,45 @@ static void setLimitsForBinOp(const BinaryOperator &BO, APInt &Lower,
   unsigned Width = Lower.getBitWidth();
   const APInt *C;
   switch (BO.getOpcode()) {
-  case Instruction::Add:
-    if (match(BO.getOperand(1), m_APInt(C)) && !C->isZero()) {
+  case Instruction::Sub:
+    if (match(BO.getOperand(0), m_APInt(C))) {
       bool HasNSW = IIQ.hasNoSignedWrap(&BO);
       bool HasNUW = IIQ.hasNoUnsignedWrap(&BO);
 
       // If the caller expects a signed compare, then try to use a signed range.
       // Otherwise if both no-wraps are set, use the unsigned range because it
       // is never larger than the signed range. Example:
-      // "add nuw nsw i8 X, -2" is unsigned [254,255] vs. signed [-128, 125].
+      // "sub nuw nsw i8 -2, x" is unsigned [0, 254] vs. signed [-128, 126].
+      // "sub nuw nsw i8 2, x" is unsigned [0, 2] vs. signed [-125, 127].
+      if (PreferSignedRange && HasNSW && HasNUW)
+        HasNUW = false;
+
+      if (HasNUW) {
+        // 'sub nuw c, x' produces [0, C].
+        Upper = *C + 1;
+      } else if (HasNSW) {
+        if (C->isNegative()) {
+          // 'sub nsw -C, x' produces [SINT_MIN, -C - SINT_MIN].
+          Lower = APInt::getSignedMinValue(Width);
+          Upper = *C - APInt::getSignedMaxValue(Width);
+        } else {
+          // Note that sub 0, INT_MIN is not NSW. It techically is a signed wrap
+          // 'sub nsw C, x' produces [C - SINT_MAX, SINT_MAX].
+          Lower = *C - APInt::getSignedMaxValue(Width);
+          Upper = APInt::getSignedMinValue(Width);
+        }
+      }
+    }
+    break;
+  case Instruction::Add:
+    if (match(BO.getOperand(1), m_APInt(C)) && !C->isZero()) {
+      bool HasNSW = IIQ.hasNoSignedWrap(&BO);
+      bool HasNUW = IIQ.hasNoUnsignedWrap(&BO);
+
+      // If the caller expects a signed compare, then try to use a signed
+      // range. Otherwise if both no-wraps are set, use the unsigned range
+      // because it is never larger than the signed range. Example: "add nuw
+      // nsw i8 X, -2" is unsigned [254,255] vs. signed [-128, 125].
       if (PreferSignedRange && HasNSW && HasNUW)
         HasNUW = false;
 

llvm/test/Transforms/InstCombine/div.ll

@@ -494,9 +494,7 @@ define <2 x i8> @sdiv_exact_negated_dividend_constant_divisor_vec_splat(<2 x i8>
 
 define i8 @sdiv_negated_dividend_constant_divisor_smin(i8 %x) {
 ; CHECK-LABEL: @sdiv_negated_dividend_constant_divisor_smin(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i8 [[X:%.*]], -128
-; CHECK-NEXT:    [[D:%.*]] = zext i1 [[TMP1]] to i8
-; CHECK-NEXT:    ret i8 [[D]]
+; CHECK-NEXT:    ret i8 0
 ;
   %neg = sub nsw i8 0, %x
   %d = sdiv i8 %neg, -128
@@ -505,9 +503,7 @@ define i8 @sdiv_negated_dividend_constant_divisor_smin(i8 %x) {
 
 define <2 x i8> @sdiv_negated_dividend_constant_divisor_vec_splat_smin(<2 x i8> %x) {
 ; CHECK-LABEL: @sdiv_negated_dividend_constant_divisor_vec_splat_smin(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq <2 x i8> [[X:%.*]], splat (i8 -128)
-; CHECK-NEXT:    [[D:%.*]] = zext <2 x i1> [[TMP1]] to <2 x i8>
-; CHECK-NEXT:    ret <2 x i8> [[D]]
+; CHECK-NEXT:    ret <2 x i8> zeroinitializer
 ;
   %neg = sub nsw <2 x i8> zeroinitializer, %x
   %d = sdiv <2 x i8> %neg, <i8 -128, i8 -128>

llvm/test/Transforms/InstCombine/icmp-sub.ll

@@ -290,8 +290,7 @@ define i1 @subC_nsw_ne(i32 %x) {
 ; CHECK-LABEL: @subC_nsw_ne(
 ; CHECK-NEXT:    [[SUBX:%.*]] = sub nsw i32 -2147483647, [[X:%.*]]
 ; CHECK-NEXT:    call void @use(i32 [[SUBX]])
-; CHECK-NEXT:    [[R:%.*]] = icmp ne i32 [[X]], 2147483603
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 true
 ;
   %subx = sub nsw i32 -2147483647, %x
   call void @use(i32 %subx)