[AArch64] Don't look at type size for scalable types in isExtFreeImpl

[dmgreen]

Summary

This fixes one of those 'Request for a fixed element count on a scalable object' errors in the AArch64 isExtFreeImpl method, where the uses of a sext are checked to see if the instruction can be considered free. https://godbolt.org/z/debYP9c4G

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

case Instruction::GetElementPtr: {
       gep_type_iterator GTI = gep_type_begin(Instr);
       auto &DL = Ext->getModule()->getDataLayout();
       std::advance(GTI, U.getOperandNo()-1);
       Type *IdxTy = GTI.getIndexedType();
       // This extension will end up with a shift because of the scaling factor.
       // 8-bit sized types have a scaling factor of 1, thus a shift amount of 0.
       // Get the shift amount based on the scaling factor:
       // log2(sizeof(IdxTy)) - log2(8).
+      if (IdxTy->isScalableTy())
+        return false;
       uint64_t ShiftAmt =
           llvm::countr_zero(DL.getTypeStoreSizeInBits(IdxTy).getFixedValue()) -
           3;
       // Is the constant foldable in the shift of the addressing mode?
       // I.e., shift amount is between 1 and 4 inclusive.
       if (ShiftAmt == 0 || ShiftAmt > 4)
         return false;
       break;

llvm/test/Analysis/CostModel/AArch64/cost-scalable-vector-gep.ll

+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 2
 ; RUN: opt -passes="print<cost-model>" 2>&1 -disable-output -mtriple=aarch64--linux-gnu -mattr=+sve < %s | FileCheck %s
 
 ; This regression test is verifying that a GEP instruction performed on a
 ; scalable vector does not produce a 'assumption that TypeSize is not scalable'
 ; warning when performing cost analysis.
 
 target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
 
-; CHECK: Cost Model: Found an estimated cost of 1 for instruction:   %retval = getelementptr
 define ptr @gep_scalable_vector(ptr %ptr) {
+; CHECK-LABEL: 'gep_scalable_vector'
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %retval = getelementptr <vscale x 16 x i8>, ptr %ptr, i32 2
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: ret ptr %retval
+;
   %retval = getelementptr <vscale x 16 x i8>, ptr %ptr, i32 2
   ret ptr %retval
 }
+
+define ptr @sext_gep(ptr %p, i32 %a) {
+; CHECK-LABEL: 'sext_gep'
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %b = sext i32 %a to i64
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %r = getelementptr <vscale x 8 x half>, ptr %p, i64 %b
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: ret ptr %r
+;
+  %b = sext i32 %a to i64
+  %r = getelementptr <vscale x 8 x half>, ptr %p, i64 %b
+  ret ptr %r
+}