[docs] [NFC] Clarify the datalayout documentation

[woodruffw]

Summary

This patch fixes a couple of small oversights in the documentation for the datalayout specification:

• The v and f specifications are subject to the same constraints on <size> as i is.
• The p[n] specification didn't mark <idx> as optional, despite being documented and parsed as such.
• Similarly, none of the alignment specifications require <pref>.

[woodruffw]

Also clarify that <pref> is optional in pointer alignment specifications.

[woodruffw]

Mark <pref> as optional in all of the alignment specs, since the DataLayout parser treats it as optional.

[woodruffw]

Ping for review on this.

[woodruffw]

Another gentle ping for review.

[fhahn]

Looks good to me in general, thanks for updating the docs! Perhaps @t.p.northover has any further suggestions?

[nikic]

llvm/docs/LangRef.rst

2563

Pre-existing, but there seems to be some words missing here. "is the size of the index that is used"?

[woodruffw]

Fixed awkward language.

llvm/docs/LangRef.rst

2563

Yeah, that reads awkwardly to me. Fixed.

[xgupta]

committed in https://reviews.llvm.org/rG86a4a93a1c444f297c2dd92815b4bf9093dfde8e. Thanks @woodruffw.

llvm/docs/LangRef.rst

 ``A<address space>``
     Specifies the address space of objects created by '``alloca``'.
     Defaults to the default address space of 0.
-``p[n]:<size>:<abi>:<pref>:<idx>``
+``p[n]:<size>:<abi>[:<pref>][:<idx>]``
     This specifies the *size* of a pointer and its ``<abi>`` and
-    ``<pref>``\erred alignments for address space ``n``. The fourth parameter
-    ``<idx>`` is a size of index that used for address calculation. If not
+    ``<pref>``\erred alignments for address space ``n``. ``<pref>`` is optional
+    and defaults to ``<abi>``. The fourth parameter ``<idx>`` is a size of
+    index that used for address calculation. If not
     specified, the default index size is equal to the pointer size. All sizes
     are in bits. The address space, ``n``, is optional, and if not specified,
     denotes the default address space 0. The value of ``n`` must be
     in the range [1,2^23).
-``i<size>:<abi>:<pref>``
+``i<size>:<abi>[:<pref>]``
     This specifies the alignment for an integer type of a given bit
     ``<size>``. The value of ``<size>`` must be in the range [1,2^23).
-``v<size>:<abi>:<pref>``
+    ``<pref>`` is optional and defaults to ``<abi>``.
+``v<size>:<abi>[:<pref>]``
     This specifies the alignment for a vector type of a given bit
-    ``<size>``.
-``f<size>:<abi>:<pref>``
+    ``<size>``. The value of ``<size>`` must be in the range [1,2^23).
+    ``<pref>`` is optional and defaults to ``<abi>``.
+``f<size>:<abi>[:<pref>]``
     This specifies the alignment for a floating-point type of a given bit
     ``<size>``. Only values of ``<size>`` that are supported by the target
     will work. 32 (float) and 64 (double) are supported on all targets; 80
     or 128 (different flavors of long double) are also supported on some
-    targets.
-``a:<abi>:<pref>``
+    targets. The value of ``<size>`` must be in the range [1,2^23).
+    ``<pref>`` is optional and defaults to ``<abi>``.
+``a:<abi>[:<pref>]``
     This specifies the alignment for an object of aggregate type.
+    ``<pref>`` is optional and defaults to ``<abi>``.
 ``F<type><abi>``
     This specifies the alignment for function pointers.
     The options for ``<type>`` are: