[SCEV] Do not strengthen nuw/nsw flags during get[Zero,Sign]ExtendedExpr.

[fhahn]

Summary

Modifying AddRecs when constructing other expressions can lead to
surprising changes. It also seems like it is not really beneficial i
most cases.

At the moment, there's a single regression, but we still might be able
to improve the flags at AddRec construction.

Might help with the issue discussed in D143409.

[mkazantsev]

llvm/test/Transforms/IndVarSimplify/lftr-reuse.ll

149

Put a TODO before this test so that we don't forget it?

[nikic]

There are some test failures.

[fhahn]

It looks like the test failures were from earlier runs, possibly with a missing dependency. I re-run them and they came back clear (on Windows, ClangdTests.exe/IncludeCleaner/GenerateMissingHeaderDiags failed, but that's very likely unrelated)

[fhahn]

llvm/test/Transforms/IndVarSimplify/lftr-reuse.ll

149

Thanks, added in the committed version!

llvm/lib/Analysis/ScalarEvolution.cpp

const SCEV *ScalarEvolution::getZeroExtendExprImpl(const SCEV *Op, Type *Ty,
   // this:  for (unsigned char X = 0; X < 100; ++X) { int Y = X; }
   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(Op))
     if (AR->isAffine()) {
       const SCEV *Start = AR->getStart();
       const SCEV *Step = AR->getStepRecurrence(*this);
       unsigned BitWidth = getTypeSizeInBits(AR->getType());
       const Loop *L = AR->getLoop();
 
-      if (!AR->hasNoUnsignedWrap()) {
-        auto NewFlags = proveNoWrapViaConstantRanges(AR);
-        setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR), NewFlags);
-      }
-
       // If we have special knowledge that this addrec won't overflow,
       // we don't need to do any further analysis.
       if (AR->hasNoUnsignedWrap()) {
         Start =
             getExtendAddRecStart<SCEVZeroExtendExpr>(AR, Ty, this, Depth + 1);
         Step = getZeroExtendExpr(Step, Ty, Depth + 1);
         return getAddRecExpr(Start, Step, L, AR->getNoWrapFlags());
       }
const SCEV *ScalarEvolution::getSignExtendExprImpl(const SCEV *Op, Type *Ty,
   // this:  for (signed char X = 0; X < 100; ++X) { int Y = X; }
   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(Op))
     if (AR->isAffine()) {
       const SCEV *Start = AR->getStart();
       const SCEV *Step = AR->getStepRecurrence(*this);
       unsigned BitWidth = getTypeSizeInBits(AR->getType());
       const Loop *L = AR->getLoop();
 
-      if (!AR->hasNoSignedWrap()) {
-        auto NewFlags = proveNoWrapViaConstantRanges(AR);
-        setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR), NewFlags);
-      }
-
       // If we have special knowledge that this addrec won't overflow,
       // we don't need to do any further analysis.
       if (AR->hasNoSignedWrap()) {
         Start =
             getExtendAddRecStart<SCEVSignExtendExpr>(AR, Ty, this, Depth + 1);
         Step = getSignExtendExpr(Step, Ty, Depth + 1);
         return getAddRecExpr(Start, Step, L, SCEV::FlagNSW);
       }

llvm/test/Transforms/IndVarSimplify/lftr-reuse.ll

 ; CHECK-NEXT:    [[INDVARS_IV:%.*]] = phi i64 [ 0, [[LOOP_PREHEADER]] ], [ [[INDVARS_IV_NEXT:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[TMP1:%.*]] = add nsw i64 [[INDVARS_IV]], [[INDVARS_IV2]]
 ; CHECK-NEXT:    [[MATRIXP:%.*]] = getelementptr inbounds [0 x double], ptr [[MATRIX:%.*]], i32 0, i64 [[TMP1]]
 ; CHECK-NEXT:    [[V1:%.*]] = load double, ptr [[MATRIXP]], align 8
 ; CHECK-NEXT:    call void @use(double [[V1]])
 ; CHECK-NEXT:    [[VECTORP:%.*]] = getelementptr inbounds [0 x double], ptr [[VECTOR:%.*]], i32 0, i64 [[INDVARS_IV2]]
 ; CHECK-NEXT:    [[V2:%.*]] = load double, ptr [[VECTORP]], align 8
 ; CHECK-NEXT:    call void @use(double [[V2]])
-; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add nsw i64 [[INDVARS_IV]], [[TMP0]]
+; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add i64 [[INDVARS_IV]], [[TMP0]]
 ; CHECK-NEXT:    [[INDVARS_IV_NEXT3]] = add nuw nsw i64 [[INDVARS_IV2]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i64 [[INDVARS_IV_NEXT3]], [[WIDE_TRIP_COUNT]]
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[RETURN_LOOPEXIT:%.*]]
 ; CHECK:       return.loopexit:
 ; CHECK-NEXT:    br label [[RETURN]]
 ; CHECK:       return:
 ; CHECK-NEXT:    ret void
 ;