[llvm] [LoopIdiom] Add a range attribute to formed memset/memcpy/memmove (PR #226801)

[Nathan Corbyn via llvm-commits]

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@@ -1089,6 +1089,41 @@ static const SCEV *getNumBytes(const SCEV *BECount, Type *IntPtr,
                         SCEV::FlagNUW);
 }
 
+/// Add a range to newly formed memset/memmove/memcpy intrinsic with an upper
+/// bound derived from the loop's constant max trip count.
+static void addRangeAttrFromTripCount(CallInst *NewCall, unsigned ArgNo,
+                                      uint64_t ElemsPerIter,
+                                      const SCEV *BECount, Loop *L,
+                                      ScalarEvolution *SE) {
+  Value *Len = NewCall->getArgOperand(ArgNo);
+  if (isa<Constant>(Len)) return;
+
+  // Two upper bounds:
+  // (1) constant max backedge-taken count
+  const APInt *Max1;
+  if (!match(SE->getConstantMaxBackedgeTakenCount(L), m_scev_APInt(Max1)))
+    return;
+
+  // (2) loop guards (new call is in preheader, so guards dominate the call)
+  APInt Max2 = SE->getUnsignedRangeMax(SE->applyLoopGuards(BECount, L));
+
+  // Use the tighter bound
+  unsigned MaxWidth = std::max(Max1->getBitWidth(), Max2.getBitWidth());
+  APInt MaxBTC = APIntOps::umin(Max1->zext(MaxWidth), Max2.zext(MaxWidth));
+
+  // Bail if bound beyond bitwidth, overflows, or is MaxValue
+  unsigned BW = Len->getType()->getIntegerBitWidth();
+  if (MaxBTC.getActiveBits() >= BW) return;
+  APInt MaxTripCount = MaxBTC.zext(BW) + 1;
----------------
cofibrant wrote:

I don't understand this. I don't think there's any guarantee at this point that `MaxBTC.getBitWidth() <= BW` so why do we care if we overflow `BW`? To make sure `MaxBTC` is expressible with `BW` bits? If so, `MaxBTC.getActiveBits() >= BW` seems like the wrong way to assert this: I understand this guarantees the add won't overflow, but `MaxBTC` could still have a very large bitwidth with only a few high bits active, leaving the value still outside of the range expressible by `BW`.

https://github.com/llvm/llvm-project/pull/226801