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

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.

Differential Revision: https://reviews.llvm.org/D135894

Author: preames

clang/lib/Basic/Targets/RISCV.cpp

@@ -246,6 +246,19 @@ bool RISCVTargetInfo::initFeatureMap(
   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;

clang/lib/Basic/Targets/RISCV.h

@@ -90,6 +90,9 @@ class RISCVTargetInfo : public TargetInfo {
                  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,

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

@@ -0,0 +1,19 @@
+// 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) {}