[AArch64] Remove FeatureUseScalarIncVL

FeatureUseScalarIncVL is a tuning feature, used to control whether addvl or
add+cnt is used. It was previously added as a dependency for FeatureSVE2, an
architecture feature but this can be seen as a layering violation. The main
disadvantage is that -use-scalar-inc-vl cannot be used without disabling sve2
and all dependant features.

This patch now replaces that with an option that if unset defaults to hasSVE ||
hasSME, but is otherwise overriden by the option. The hope is that no cpus will
rely on the tuning feature (or we can readdit if needed.

Author: davemgreen

llvm/lib/Target/AArch64/AArch64Features.td

@@ -358,12 +358,9 @@ def FeatureTHE : ExtensionWithMArch<"the", "THE", "FEAT_THE",
 //  Armv9.0 Architecture Extensions
 //===----------------------------------------------------------------------===//
 
-def FeatureUseScalarIncVL : SubtargetFeature<"use-scalar-inc-vl",
-  "UseScalarIncVL", "true", "Prefer inc/dec over add+cnt">;
-
 def FeatureSVE2 : ExtensionWithMArch<"sve2", "SVE2", "FEAT_SVE2",
   "Enable Scalable Vector Extension 2 (SVE2) instructions",
-  [FeatureSVE, FeatureUseScalarIncVL]>;
+  [FeatureSVE]>;
 
 def FeatureSVE2AES : ExtensionWithMArch<"sve2-aes", "SVE2AES",
   "FEAT_SVE_AES, FEAT_SVE_PMULL128",
@@ -403,7 +400,7 @@ def FeatureRME : Extension<"rme", "RME", "FEAT_RME",
   "Enable Realm Management Extension">;
 
 def FeatureSME : ExtensionWithMArch<"sme", "SME", "FEAT_SME",
-  "Enable Scalable Matrix Extension (SME)", [FeatureBF16, FeatureUseScalarIncVL]>;
+  "Enable Scalable Matrix Extension (SME)", [FeatureBF16]>;
 
 def FeatureSMEF64F64 : ExtensionWithMArch<"sme-f64f64", "SMEF64F64", "FEAT_SME_F64F64",
   "Enable Scalable Matrix Extension (SME) F64F64 instructions", [FeatureSME]>;

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

@@ -93,6 +93,10 @@ static cl::opt<bool>
                                  cl::init(false), cl::Hidden,
                                  cl::desc("Enable subreg liveness tracking"));
 
+static cl::opt<bool>
+    UseScalarIncVL("sve-use-scalar-inc-vl", cl::init(false), cl::Hidden,
+                   cl::desc("Prefer add+cnt over addvl/inc/dec"));
+
 unsigned AArch64Subtarget::getVectorInsertExtractBaseCost() const {
   if (OverrideVectorInsertExtractBaseCost.getNumOccurrences() > 0)
     return OverrideVectorInsertExtractBaseCost;
@@ -575,6 +579,14 @@ void AArch64Subtarget::mirFileLoaded(MachineFunction &MF) const {
 
 bool AArch64Subtarget::useAA() const { return UseAA; }
 
+bool AArch64Subtarget::useScalarIncVL() const {
+  // If SVE2 or SME is present (we are not SVE-1 only) and UseScalarIncVL
+  // is not otherwise set, enable it by default.
+  if (UseScalarIncVL.getNumOccurrences())
+    return UseScalarIncVL;
+  return hasSVE2() || hasSME();
+}
+
 // If return address signing is enabled, tail calls are emitted as follows:
 //
 // ```

llvm/lib/Target/AArch64/AArch64Subtarget.h

@@ -417,6 +417,10 @@ class AArch64Subtarget final : public AArch64GenSubtargetInfo {
     return DefaultSVETFOpts;
   }
 
+  /// Returns true to use the addvl/inc/dec instructions, as opposed to separate
+  /// add + cnt instructions.
+  bool useScalarIncVL() const;
+
   const char* getChkStkName() const {
     if (isWindowsArm64EC())
       return "#__chkstk_arm64ec";

llvm/test/CodeGen/AArch64/sve-intrinsics-counting-elems-i32.ll

@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve < %s | FileCheck %s -check-prefix=NO_SCALAR_INC
-; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve -mattr=+use-scalar-inc-vl < %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve -sve-use-scalar-inc-vl=true < %s | FileCheck %s
 ; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve2 < %s | FileCheck %s
 
 ; INCB

llvm/test/CodeGen/AArch64/sve-intrinsics-counting-elems.ll

@@ -1,8 +1,10 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve < %s | FileCheck %s
-; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve -mattr=+use-scalar-inc-vl < %s | FileCheck %s -check-prefix=USE_SCALAR_INC
+; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve -sve-use-scalar-inc-vl=true < %s | FileCheck %s -check-prefix=USE_SCALAR_INC
 ; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve2 < %s | FileCheck %s -check-prefix=USE_SCALAR_INC
 ; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sme -force-streaming < %s | FileCheck %s -check-prefix=USE_SCALAR_INC
+; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve2 -sve-use-scalar-inc-vl=false < %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sme -sve-use-scalar-inc-vl=false -force-streaming < %s | FileCheck %s
 
 ;
 ; CNTB

llvm/test/CodeGen/AArch64/sve-vl-arith.ll

@@ -1,7 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 2
 ; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve -verify-machineinstrs < %s | FileCheck %s -check-prefix=NO_SCALAR_INC
-; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve -mattr=+use-scalar-inc-vl -verify-machineinstrs < %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve -sve-use-scalar-inc-vl=true -verify-machineinstrs < %s | FileCheck %s
 ; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve2 -verify-machineinstrs < %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve2 -sve-use-scalar-inc-vl=false -verify-machineinstrs < %s | FileCheck %s -check-prefix=NO_SCALAR_INC
 
 define <vscale x 8 x i16> @inch_vec(<vscale x 8 x i16> %a) {
 ; NO_SCALAR_INC-LABEL: inch_vec: