[llvm] [LangRef/DataLayout] Spell out requirements for alignment values (PR #104705)

[Sergei Barannikov via llvm-commits]

https://github.com/s-barannikov updated https://github.com/llvm/llvm-project/pull/104705

>From 7c8791337aeb57e0e457079ecac949422c6e9f69 Mon Sep 17 00:00:00 2001
From: Sergei Barannikov <barannikov88 at gmail.com>
Date: Thu, 15 Aug 2024 23:16:55 +0300
Subject: [PATCH 1/2] [LangRef/DataLayout] Spell out requirements for alignment
 values

For 'p' the added wording matches the implementation.

For 'i', 'f', 'v' the implementation also allows 0 for <pref> component,
making 'i16:16:0' valid, for example. 'Fi0', 'Fn0' and 'S0' are also
currently accepted. This is likely unintentional. There are no tests
in the codebase that rely on this behavior, so the added wording
prohibits zero alignments for these specifications.

For 'a', the implementation currently allows 0 for both <abi> and <pref>
components. The added wording prohibits specifying zero for <pref>
with the same justification as above. Zero <abi> is used in tests,
and the example at the end of the section suggests that this is valid,
so that is left unchanged.

This effectively prohibits zero alignments everywhere except for
the <abi> component of aggregate specification.
---
 llvm/docs/LangRef.rst | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 5e5e9b9e8a93b1..ea4fb6030161aa 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -3030,8 +3030,10 @@ as follows:
 ``S<size>``
     Specifies the natural alignment of the stack in bits. Alignment
     promotion of stack variables is limited to the natural stack
-    alignment to avoid dynamic stack realignment. The stack alignment
-    must be a multiple of 8-bits. If omitted, the natural stack
+    alignment to avoid dynamic stack realignment.
+    The stack alignment must be must be in the range [1,2^16)
+    and must be a power of two times the byte width.
+    If omitted, the natural stack
     alignment defaults to "unspecified", which does not prevent any
     alignment promotions.
 ``P<address space>``
@@ -3065,16 +3067,22 @@ as follows:
     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 [1,2^24).
+    The values of ``<abi>`` and ``<pref>`` must be in the range [1,2^16)
+    and must be powers of two times the byte width.
 ``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 [1,2^24).
     ``<pref>`` is optional and defaults to ``<abi>``.
+    The values of ``<abi>`` and ``<pref>`` must be in the range [1,2^16)
+    and must be powers of two times the byte width.
     For ``i8``, the ``<abi>`` value must equal 8,
     that is, ``i8`` must be naturally aligned.
 ``v<size>:<abi>[:<pref>]``
     This specifies the alignment for a vector type of a given bit
     ``<size>``. The value of ``<size>`` must be in the range [1,2^24).
     ``<pref>`` is optional and defaults to ``<abi>``.
+    The values of ``<abi>`` and ``<pref>`` must be in the range [1,2^16)
+    and must be powers of two times the byte width.
 ``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
@@ -3082,9 +3090,14 @@ as follows:
     or 128 (different flavors of long double) are also supported on some
     targets. The value of ``<size>`` must be in the range [1,2^24).
     ``<pref>`` is optional and defaults to ``<abi>``.
+    The values of ``<abi>`` and ``<pref>`` must be in the range [1,2^16)
+    and must be powers of two times the byte width.
 ``a:<abi>[:<pref>]``
     This specifies the alignment for an object of aggregate type.
     ``<pref>`` is optional and defaults to ``<abi>``.
+    The values of ``<abi>`` and ``<pref>`` must be in the range [1,2^16)
+    and must be powers of two times the byte width.
+    The value of ``<<abi>>` may also be zero meaning one byte alignment.
 ``F<type><abi>``
     This specifies the alignment for function pointers.
     The options for ``<type>`` are:
@@ -3093,6 +3106,8 @@ as follows:
       of functions, and is a multiple of ``<abi>``.
     * ``n``: The alignment of function pointers is a multiple of the explicit
       alignment specified on the function, and is a multiple of ``<abi>``.
+    The values of ``<abi>`` and ``<pref>`` must be in the range [1,2^16)
+    and must be powers of two times the byte width.
 ``m:<mangling>``
     If present, specifies that llvm names are mangled in the output. Symbols
     prefixed with the mangling escape character ``\01`` are passed through

>From b5150ad054011cb56b5c7e2e0a7d0a149dfb4527 Mon Sep 17 00:00:00 2001
From: Sergei Barannikov <barannikov88 at gmail.com>
Date: Sun, 18 Aug 2024 16:37:35 +0300
Subject: [PATCH 2/2] fix typo

---
 llvm/docs/LangRef.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index ea4fb6030161aa..16ddd9b2a3720d 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -3097,7 +3097,7 @@ as follows:
     ``<pref>`` is optional and defaults to ``<abi>``.
     The values of ``<abi>`` and ``<pref>`` must be in the range [1,2^16)
     and must be powers of two times the byte width.
-    The value of ``<<abi>>` may also be zero meaning one byte alignment.
+    The value of ``<abi>`` may also be zero meaning one byte alignment.
 ``F<type><abi>``
     This specifies the alignment for function pointers.
     The options for ``<type>`` are: