[llvm] [InstCombine] Fold @llvm.experimental.get.vector.length when cnt <= max_lanes (PR #169293)

[Luke Lau via llvm-commits]

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@@ -4005,6 +4005,22 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     }
     break;
   }
+  case Intrinsic::experimental_get_vector_length: {
+    // get.vector.length(Cnt, MaxLanes) --> Cnt when Cnt <= MaxLanes
+    ConstantRange Cnt = computeConstantRangeIncludingKnownBits(
+        II->getArgOperand(0), false, SQ.getWithInstruction(II));
+    ConstantRange MaxLanes = cast<ConstantInt>(II->getArgOperand(1))
+                                 ->getValue()
+                                 .zext(Cnt.getBitWidth());
+    if (cast<ConstantInt>(II->getArgOperand(2))->getZExtValue())
+      MaxLanes = MaxLanes.multiply(
+          getVScaleRange(II->getFunction(), Cnt.getBitWidth()));
+
+    if (Cnt.icmp(CmpInst::ICMP_ULE, MaxLanes))
+      return replaceInstUsesWith(
+          *II, Builder.CreateTrunc(II->getArgOperand(0), II->getType()));
----------------
lukel97 wrote:

Thanks, done in 2f71e8a38242da1740c8820d659e0a4ea95e5f2f

https://github.com/llvm/llvm-project/pull/169293