Tighten description of ISD::BUILD_VECTOR

[cameron.mcinally]

Summary

VerifySDNode(...) in SelectionDAG.cpp shows that the operands of a BUILD_VECTOR must all be the same type. This patch cleans up the comment in ISDOpcodes.h to make that more obvious.

[craig.topper]

llvm/include/llvm/CodeGen/ISDOpcodes.h

450–451

While you're here can you drop "The number of elements is required to be a power of two." That's not true.

[cameron.mcinally]

Remove power-of-2 requirement.

[craig.topper]

LGTM

[kparzysz]

I'd move the "all types must be the same" to the end, so that it doesn't look like the exception overrides it, i.e. something like

The types of the operands must match the vector element type, except
that for integer types the operands are allowed to be of a larger type
than the element type, in which case the operands are implicitly
truncated. In any case, all operands must have the same type.

[cameron.mcinally]

Thanks, @kparzysz . I like that idea. Pushed 898065a7b879f204874820f16e4e16ea2a961de0. Would you review post-commit?

Unfortunately, I missed your quoted recommendation until just now. My email client hid it. Are you okay with it as-is?

llvm/include/llvm/CodeGen/ISDOpcodes.h

enum NodeType {
 
   /// INT = FGETSIGN(FP) - Return the sign bit of the specified floating point
   /// value as an integer 0/1 value.
   FGETSIGN,
 
   /// Returns platform specific canonical encoding of a floating point number.
   FCANONICALIZE,
 
   /// BUILD_VECTOR(ELT0, ELT1, ELT2, ELT3,...) - Return a fixed-width vector
-  /// with the specified, possibly variable, elements.  The number of elements
-  /// is required to be a power of two. The types of the operands must all be
-  /// the same and must match the vector element type, except that integer types
-  /// are allowed to be larger than the element type, in which case the operands
-  /// are implicitly truncated.
+  /// with the specified, possibly variable, elements. The types of the
+  /// operands must all be the same. The types of the operands must match the
+  /// vector element type, except that integer types are allowed to be larger
+  /// than the element type, in which case the operands are implicitly
+  /// truncated.
   BUILD_VECTOR,
 
   /// INSERT_VECTOR_ELT(VECTOR, VAL, IDX) - Returns VECTOR with the element
   /// at IDX replaced with VAL. If the type of VAL is larger than the vector
   /// element type then VAL is truncated before replacement.
   ///
   /// If VECTOR is a scalable vector, then IDX may be larger than the minimum
   /// vector width. IDX is not first scaled by the runtime scaling factor of