[LoopVectorizer] Add a -force-vscale-for-tuning override option.

It can be useful for debugging and tuning to be able to alter the
VScaleForTuning. This adds a quick option to the vectorizer for it

Author: davemgreen

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -294,6 +294,10 @@ static cl::opt<bool> ForceTargetSupportsScalableVectors(
         "Pretend that scalable vectors are supported, even if the target does "
         "not support them. This flag should only be used for testing."));
 
+static cl::opt<unsigned> VScaleForTuningOpt(
+    "force-vscale-for-tuning", cl::Hidden,
+    cl::desc("Force a vscale for tuning factor in the loop vectorizer"));
+
 static cl::opt<unsigned> SmallLoopCost(
     "small-loop-cost", cl::init(20), cl::Hidden,
     cl::desc(
@@ -1463,6 +1467,11 @@ class LoopVectorizationCostModel {
   /// vscale_range.min == vscale_range.max then return vscale_range.max, else
   /// return the value returned by the corresponding TTI method.
   void initializeVScaleForTuning() {
+    if (VScaleForTuningOpt.getNumOccurrences()) {
+      VScaleForTuning = VScaleForTuningOpt;
+      return;
+    }
+
     const Function *Fn = TheLoop->getHeader()->getParent();
     if (Fn->hasFnAttribute(Attribute::VScaleRange)) {
       auto Attr = Fn->getFnAttribute(Attribute::VScaleRange);

llvm/test/Transforms/LoopVectorize/AArch64/scalable-vectorization-cost-tuning.ll

@@ -7,6 +7,10 @@
 ; RUN:     -force-target-instruction-cost=1 -passes=loop-vectorize -S -debug-only=loop-vectorize --disable-output < %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefixes=VSCALEFORTUNING1
 
+; RUN: opt -mtriple=aarch64 -mattr=+sve -mcpu=generic -force-vscale-for-tuning=2 \
+; RUN:     -force-target-instruction-cost=1 -passes=loop-vectorize -S -debug-only=loop-vectorize --disable-output < %s 2>&1 \
+; RUN:     | FileCheck %s --check-prefixes=VSCALEFORTUNING2
+
 ; RUN: opt -mtriple=aarch64 -mcpu=neoverse-v1 \
 ; RUN:     -force-target-instruction-cost=1 -passes=loop-vectorize -S -debug-only=loop-vectorize --disable-output < %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefixes=VSCALEFORTUNING2