[clang][RISCV] Set vscale_range attribute based on presence of "v" extension

[reames]

Summary

This follows the path that AArch64 SVE has taken. Doing this via a function attribute set in the frontend is basically a workaround for the fact that several analyzes which need the information (i.e. known bits, lvi, scev) can't easily use TTI without significant amounts of plumbing changes.

This patch hard codes "v" numbers, and directly follows the SVE precedent as a result. In a follow up, I hope to drive this from RISCVISAInfo.h/cpp instead, but the MinVLen number being returned from that interface seemed to always be 0 (which is wrong), and I haven't figured out what's going wrong there.

[craig.topper]

There's also this patch from @frasercrmck https://reviews.llvm.org/D107290

[reames]

In D135894#3856229, @craig.topper wrote:

There's also this patch from @frasercrmck https://reviews.llvm.org/D107290

So there is; had not seen that. Looking at it, that patch seems to roll several items together. I'd strongly prefer if we worked incrementally here. I think we do need to merge the frontend and backend handling here, but we can we work one step at a time?

Steps as I currently see them:
Step 1 - get basic support for "v", to unblock practical codegen
Step 2 - expose the necessary constants for frontend use, and rephrase in terms of VLEN
Step 3 - figure out why getMinVLen on RISCVISAInfo is currently always returning 0, fix that
Step 4 - revisit question if whether the result replaces TTI callback (I don't think it does, but don't want to speculate too much now.)

I'm willing to commit my time through at least step 3, and possibly beyond.

[craig.topper]

clang/lib/Basic/Targets/RISCV.cpp

256

Minimum

[reames]

fix typo

[craig.topper]

LGTM

clang/lib/Basic/Targets/RISCV.h

public:
 
   std::string convertConstraint(const char *&Constraint) const override;
 
   bool
   initFeatureMap(llvm::StringMap<bool> &Features, DiagnosticsEngine &Diags,
                  StringRef CPU,
                  const std::vector<std::string> &FeaturesVec) const override;
 
+  Optional<std::pair<unsigned, unsigned>>
+  getVScaleRange(const LangOptions &LangOpts) const override;
+
   bool hasFeature(StringRef Feature) const override;
 
   bool handleTargetFeatures(std::vector<std::string> &Features,
                             DiagnosticsEngine &Diags) override;
 
   bool hasBitIntType() const override { return true; }
 
   bool useFP16ConversionIntrinsics() const override {

clang/lib/Basic/Targets/RISCV.cpp

bool RISCVTargetInfo::initFeatureMap(
   // Add non-ISA features like `relax` and `save-restore` back
   for (const std::string &Feature : FeaturesVec)
     if (!llvm::is_contained(ImpliedFeatures, Feature))
       ImpliedFeatures.push_back(Feature);
 
   return TargetInfo::initFeatureMap(Features, Diags, CPU, ImpliedFeatures);
 }
 
+Optional<std::pair<unsigned, unsigned>>
+RISCVTargetInfo::getVScaleRange(const LangOptions &LangOpts) const {
+  if (LangOpts.VScaleMin || LangOpts.VScaleMax)
+    return std::pair<unsigned, unsigned>(
+        LangOpts.VScaleMin ? LangOpts.VScaleMin : 1, LangOpts.VScaleMax);
+
+  if (hasFeature("v"))
+    // Minimum VLEN=128, Maximum VLEN=64k, and RISCV::RVVBitsPerBlock is 64.
+    return std::pair<unsigned, unsigned>(2, 1024);
+
+  return None;
+}
+
 /// Return true if has this feature, need to sync with handleTargetFeatures.
 bool RISCVTargetInfo::hasFeature(StringRef Feature) const {
   bool Is64Bit = getTriple().getArch() == llvm::Triple::riscv64;
   auto Result = llvm::StringSwitch<Optional<bool>>(Feature)
                     .Case("riscv", true)
                     .Case("riscv32", !Is64Bit)
                     .Case("riscv64", Is64Bit)
                     .Case("32bit", !Is64Bit)

clang/test/CodeGen/riscv-vector-bits-vscale-range.c

+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=1 -mvscale-max=1 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=1
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=2 -mvscale-max=2 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=2
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=4 -mvscale-max=4 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=4
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=8 -mvscale-max=8 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=8
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=16 -mvscale-max=16 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=16
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=1 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=1 --check-prefix=CHECK-NOMAX
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=2 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=2 --check-prefix=CHECK-NOMAX
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=4 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=4 --check-prefix=CHECK-NOMAX
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=8 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=8 --check-prefix=CHECK-NOMAX
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=16 -S -emit-llvm -o - %s | FileCheck %s -D#VBITS=16 --check-prefix=CHECK-NOMAX
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -mvscale-min=1 -mvscale-max=0 -S -emit-llvm -o - %s | FileCheck %s --check-prefix=CHECK-UNBOUNDED
+// RUN: %clang_cc1 -triple riscv64-none-linux-gnu -target-feature +v -S -emit-llvm -o - %s | FileCheck %s --check-prefix=CHECK-NONE
+
+// CHECK-LABEL: @func() #0
+// CHECK: attributes #0 = { {{.*}} vscale_range([[#VBITS]],[[#VBITS]]) {{.*}} }
+// CHECK-NOMAX: attributes #0 = { {{.*}} vscale_range([[#VBITS]],0) {{.*}} }
+// CHECK-UNBOUNDED: attributes #0 = { {{.*}} vscale_range(1,0) {{.*}} }
+// CHECK-NONE: attributes #0 = { {{.*}} vscale_range(2,1024) {{.*}} }
+void func(void) {}