[llvm] [SCEV] Rework wrap-flag-inference for sext-addrec (PR #217362)

[Nikita Popov via llvm-commits]

================
@@ -2044,91 +2044,17 @@ const SCEV *ScalarEvolution::getSignExtendExprImpl(SCEVUse Op, Type *Ty,
   // operands (often constants).  This allows analysis of something like
   // this:  for (signed char X = 0; X < 100; ++X) { int Y = X; }
   if (match(Op, m_scev_AffineAddRec(m_SCEV(Start), m_SCEV(Step), m_Loop(L)))) {
+    // Redo the AddRec check, computing nsw this time.
     const auto *AR = cast<SCEVAddRecExpr>(Op);
-    unsigned BitWidth = getTypeSizeInBits(AR->getType());
-
-    // The no-signed-wrap case is handled before the uniquing lookup above.
-
-    // Check whether the backedge-taken count is SCEVCouldNotCompute.
-    // Note that this serves two purposes: It filters out loops that are
-    // simply not analyzable, and it covers the case where this code is
-    // being called from within backedge-taken count analysis, such that
-    // attempting to ask for the backedge-taken count would likely result
-    // in infinite recursion. In the later case, the analysis code will
-    // cope with a conservative value, and it will take care to purge
-    // that value once it has finished.
-    const SCEV *MaxBECount = getConstantMaxBackedgeTakenCount(L);
-    if (!isa<SCEVCouldNotCompute>(MaxBECount)) {
-      // Manually compute the final value for AR, checking for
-      // overflow.
-
-      // Check whether the backedge-taken count can be losslessly casted to
-      // the addrec's type. The count is always unsigned.
-      const SCEV *CastedMaxBECount =
-          getTruncateOrZeroExtend(MaxBECount, Start->getType(), Depth);
-      const SCEV *RecastedMaxBECount = getTruncateOrZeroExtend(
-          CastedMaxBECount, MaxBECount->getType(), Depth);
-      if (MaxBECount == RecastedMaxBECount) {
-        Type *WideTy = IntegerType::get(getContext(), BitWidth * 2);
-        // Check whether Start+Step*MaxBECount has no signed overflow.
-        const SCEV *SMul =
-            getMulExpr(CastedMaxBECount, Step, SCEV::FlagAnyWrap, Depth + 1);
-        const SCEV *SAdd = getSignExtendExpr(
-            getAddExpr(Start, SMul, SCEV::FlagAnyWrap, Depth + 1), WideTy,
-            Depth + 1);
-        const SCEV *WideStart = getSignExtendExpr(Start, WideTy, Depth + 1);
-        const SCEV *WideMaxBECount =
-            getZeroExtendExpr(CastedMaxBECount, WideTy, Depth + 1);
-        const SCEV *OperandExtendedAdd =
-            getAddExpr(WideStart,
-                       getMulExpr(WideMaxBECount,
-                                  getSignExtendExpr(Step, WideTy, Depth + 1),
-                                  SCEV::FlagAnyWrap, Depth + 1),
-                       SCEV::FlagAnyWrap, Depth + 1);
-        if (SAdd == OperandExtendedAdd) {
-          // Cache knowledge of AR NSW, which is propagated to this AddRec.
-          setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR), SCEV::FlagNSW);
-          // Return the expression with the addrec on the outside.
-          Start =
-              getExtendAddRecStart<SCEVSignExtendExpr>(AR, Ty, this, Depth + 1);
-          Step = getSignExtendExpr(Step, Ty, Depth + 1);
-          return getAddRecExpr(Start, Step, L, AR->getNoWrapFlags());
-        }
-        // Similar to above, only this time treat the step value as unsigned.
-        // This covers loops that count up with an unsigned step.
-        OperandExtendedAdd =
-            getAddExpr(WideStart,
-                       getMulExpr(WideMaxBECount,
-                                  getZeroExtendExpr(Step, WideTy, Depth + 1),
-                                  SCEV::FlagAnyWrap, Depth + 1),
-                       SCEV::FlagAnyWrap, Depth + 1);
-        if (SAdd == OperandExtendedAdd) {
-          // If AR wraps around then
-          //
-          //    abs(Step) * MaxBECount > unsigned-max(AR->getType())
-          // => SAdd != OperandExtendedAdd
-          //
-          // Thus (AR is not NW => SAdd != OperandExtendedAdd) <=>
-          // (SAdd == OperandExtendedAdd => AR is NW)
-
-          setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR), SCEV::FlagNW);
-
-          // Return the expression with the addrec on the outside.
-          Start =
-              getExtendAddRecStart<SCEVSignExtendExpr>(AR, Ty, this, Depth + 1);
-          Step = getZeroExtendExpr(Step, Ty, Depth + 1);
-          return getAddRecExpr(Start, Step, L, AR->getNoWrapFlags());
-        }
-      }
-    }
-
+    inferNoWrapViaConstantRanges(AR);
----------------
nikic wrote:

This bit is less clear to me -- why do we need to rerun inferNoWrapViaConstantRanges() which already ran on construction? Is this about addrecs that get created via getAddRecExpr() rather than via IR? I guess we don't run in the inference logic for those...

Also this drops the `nw` bit leading to the test regression. Presumably on the premise that it's incorrect, but as commented on the other PRs, I don't think it's actually incorrect.

https://github.com/llvm/llvm-project/pull/217362