InstSimplify: teach simplifyICmpWithConstant about samesign

We have chosen to change ConstantRange::makeAllowedICmpRegion to respect
samesign information, noting that ConstantRange::makeExactICmpRegion
should not be modified.

Author: artagnon

llvm/include/llvm/IR/ConstantRange.h

@@ -34,14 +34,14 @@
 #include "llvm/ADT/APInt.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instruction.h"
-#include "llvm/Support/Compiler.h"
 #include <cstdint>
 
 namespace llvm {
 
 class MDNode;
 class raw_ostream;
 struct KnownBits;
+class CmpPredicate;
 
 /// This class represents a range of values.
 class [[nodiscard]] ConstantRange {
@@ -103,6 +103,11 @@ class [[nodiscard]] ConstantRange {
   static ConstantRange makeAllowedICmpRegion(CmpInst::Predicate Pred,
                                              const ConstantRange &Other);
 
+  /// Calls makeAllowedICmpRegion with a CmpPredicate, yielding an asymmetric
+  /// range when samesign information is present.
+  static ConstantRange makeAsymmetricICmpRegion(CmpPredicate Pred,
+                                                const ConstantRange &Other);
+
   /// Produce the largest range such that all values in the returned range
   /// satisfy the given predicate with all values contained within Other.
   /// Formally, this returns a subset of

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -3012,7 +3012,7 @@ static Value *simplifyICmpWithConstant(CmpPredicate Pred, Value *LHS,
   }
 
   // Rule out tautological comparisons (eg., ult 0 or uge 0).
-  ConstantRange RHS_CR = ConstantRange::makeExactICmpRegion(Pred, *C);
+  ConstantRange RHS_CR = ConstantRange::makeAsymmetricICmpRegion(Pred, *C);
   if (RHS_CR.isEmptySet())
     return ConstantInt::getFalse(ITy);
   if (RHS_CR.isFullSet())

llvm/lib/IR/ConstantRange.cpp

@@ -145,6 +145,14 @@ ConstantRange ConstantRange::makeAllowedICmpRegion(CmpInst::Predicate Pred,
   }
 }
 
+ConstantRange ConstantRange::makeAsymmetricICmpRegion(CmpPredicate Pred,
+                                                      const ConstantRange &CR) {
+  if (Pred.hasSameSign() && ICmpInst::isRelational(Pred))
+    return makeAllowedICmpRegion(Pred, CR).unionWith(makeAllowedICmpRegion(
+        ICmpInst::getFlippedSignednessPredicate(Pred), CR));
+  return makeAllowedICmpRegion(Pred, CR);
+}
+
 ConstantRange ConstantRange::makeSatisfyingICmpRegion(CmpInst::Predicate Pred,
                                                       const ConstantRange &CR) {
   // Follows from De-Morgan's laws:

llvm/test/Analysis/ValueTracking/constant-ranges.ll

@@ -180,7 +180,7 @@ define i1 @srem_posC_okay1(i8 %x) {
 
 define i1 @srem_posC_okay1_samesign(i8 %x) {
 ; CHECK-LABEL: @srem_posC_okay1_samesign(
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 true
 ;
   %val = srem i8 34, %x
   %r = icmp samesign uge i8 %val, -3
@@ -198,9 +198,7 @@ define i1 @srem_negC_okay0(i8 %x) {
 
 define i1 @srem_negC_okay0_samesign(i8 %x) {
 ; CHECK-LABEL: @srem_negC_okay0_samesign(
-; CHECK-NEXT:    [[VAL:%.*]] = srem i8 -34, [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp samesign ule i8 [[VAL]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 true
 ;
   %val = srem i8 -34, %x
   %r = icmp samesign ule i8 %val, 0
@@ -218,9 +216,7 @@ define i1 @srem_negC_okay1(i8 %x) {
 
 define i1 @srem_negC_okay1_samesign(i8 %x) {
 ; CHECK-LABEL: @srem_negC_okay1_samesign(
-; CHECK-NEXT:    [[VAL:%.*]] = srem i8 -34, [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp samesign uge i8 [[VAL]], -34
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 true
 ;
   %val = srem i8 -34, %x
   %r = icmp samesign uge i8 %val, -34