[llvm] [RISCV] Track the value range of vsetvlimax (PR #218313)

[Luke Lau via llvm-commits]

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@@ -87,6 +87,37 @@
 using namespace llvm;
 using namespace llvm::PatternMatch;
 
+// Return the architectural range for VLMAX. RISC-V defines VLEN in the range
+// [32, 65536], while vscale_range can provide tighter bounds for VLEN >= 64.
+static ConstantRange getRISCVVSetVLMaxRange(const IntrinsicInst &II,
+                                            unsigned ArgOffset = 0) {
+  unsigned Width = II.getType()->getScalarSizeInBits();
+  constexpr unsigned MinVLen = 32;
+  constexpr unsigned MaxVLen = 65536;
+  ConstantRange VLenRange(APInt(Width, MinVLen), APInt(Width, MaxVLen + 1));
+  if (II.getFunction() &&
+      II.getFunction()->hasFnAttribute(Attribute::VScaleRange)) {
+    ConstantRange VScaleRange = getVScaleRange(II.getFunction(), Width);
+    VScaleRange = VScaleRange.intersectWith(ConstantRange(
+        APInt(Width, 1), APInt(Width, MaxVLen / RISCV::RVVBitsPerBlock + 1)));
+    VLenRange = VScaleRange.multiply(
+        ConstantRange(APInt(Width, RISCV::RVVBitsPerBlock)));
+  }
+
+  auto *VSEW = dyn_cast<ConstantInt>(II.getArgOperand(ArgOffset));
+  auto *VLMULArg = dyn_cast<ConstantInt>(II.getArgOperand(ArgOffset + 1));
+  // These are immarg operands, but keep this helper conservative for malformed
+  // IR rather than asserting while performing generic value analysis.
+  if (!VSEW || !VLMULArg || VSEW->getZExtValue() > 3 ||
+      VLMULArg->getZExtValue() > 7 || VLMULArg->getZExtValue() == 4)
+    return ConstantRange::getFull(Width);
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lukel97 wrote:

I don't think malformed IR can get through here, doesn't the verifier reject it? Probably best to keep the cast and assert

https://github.com/llvm/llvm-project/pull/218313