[Doc] Refactor descriptions of `min-legal-vector-width`

[pengfei]

Summary

Based on comments from @arsenm.

[arsenm]

llvm/docs/LangRef.rst

2236–2237 ↗

(On Diff #481896)

The first sentence isn't grammatically correct. Also should not start out with the word legal

2239 ↗

(On Diff #481896)

Still not stating what legal means in this context

2240 ↗

(On Diff #481896)

Grammar?

2241 ↗

(On Diff #481896)

return types?

Also what is the behavior for aggregates? Aggregates containing vectors? Nested aggregates with vectors?

2243 ↗

(On Diff #481896)

Returnings isn't a word; "return values"?

2244–2245 ↗

(On Diff #481896)

We can't spec attributes in terms of what clang's behavior is today

[pengfei]

I want to remove the description from LangRef directly since we decided to only support this attribute in X86. WDYT?

llvm/docs/LangRef.rst

2241 ↗

(On Diff #481896)

Good question. I think we have considered such cases. See https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/CGCall.cpp#L4729-L4740

[pengfei]

Move doc to comments.

[pengfei]

clang/lib/CodeGen/CodeGenFunction.cpp

502–504

I still think the sentence is ill-formed. But I have tried my best. Welcome for suggestions.

clang/lib/CodeGen/CodeGenFunction.cpp

void CodeGenFunction::FinishFunction(SourceLocation EndLoc) {
   if (auto *VT = dyn_cast<llvm::VectorType>(CurFn->getReturnType()))
     LargestVectorWidth =
         std::max((uint64_t)LargestVectorWidth,
                  VT->getPrimitiveSizeInBits().getKnownMinSize());
 
   if (CurFnInfo->getMaxVectorWidth() > LargestVectorWidth)
     LargestVectorWidth = CurFnInfo->getMaxVectorWidth();
 
-  // Add the required-vector-width attribute. This contains the max width from:
-  // 1. min-vector-width attribute used in the source program.
+  // Add the min-legal-vector-width attribute. This contains the max width from:
+  // 1. min-legal-vector-width attribute used in the source program.
   // 2. Any builtins used that have a vector width specified.
   // 3. Values passed in and out of inline assembly.
   // 4. Width of vector arguments and return types for this function.
-  // 5. Width of vector aguments and return types for functions called by this
+  // 5. Width of vector arguments and return types for functions called by this
   //    function.
+  // This attribute is intended for X86 backend use only. The "min legal" in the
+  // name means the minimum width in bits that vector types have must be treated
+  // as legal types by code generator.
+  // Note: The attribute doesn't guarantee arbitrary vector width specified will
+  // be treated as legal type in code generator. Users are not encouraged to
+  // directly pass or return vector types out of the capacity of their targets.
   if (getContext().getTargetInfo().getTriple().isX86())
     CurFn->addFnAttr("min-legal-vector-width",
                      llvm::utostr(LargestVectorWidth));
 
   // Add vscale_range attribute if appropriate.
   Optional<std::pair<unsigned, unsigned>> VScaleRange =
       getContext().getTargetInfo().getVScaleRange(getLangOpts());
   if (VScaleRange) {