[llvm] asm clobber constrains: '~memory' allows reads and synchronization (PR #150191)

[Ralf Jung via llvm-commits]

https://github.com/RalfJung created https://github.com/llvm/llvm-project/pull/150191

Cc @nikic -- does this sound right? I was not sure what the best way is for talking about "synchronization effects".

>From 951979f4707ad4277212bdbb1ea29e38a9e44fe9 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post at ralfj.de>
Date: Wed, 23 Jul 2025 11:06:14 +0200
Subject: [PATCH] asm clobber constrains: '~memory' allows reads and
 synchronization

---
 llvm/docs/LangRef.rst | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index bac13cc0424a6..9f792fa392cca 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -5448,10 +5448,13 @@ Clobber constraints
 A clobber constraint is indicated by a "``~``" prefix. A clobber does not
 consume an input operand, nor generate an output. Clobbers cannot use any of the
 general constraint code letters -- they may use only explicit register
-constraints, e.g. "``~{eax}``". The one exception is that a clobber string of
-"``~{memory}``" indicates that the assembly writes to arbitrary undeclared
-memory locations -- not only the memory pointed to by a declared indirect
-output.
+constraints, e.g. "``~{eax}``".
+
+The one exception is that a clobber string of "``~{memory}``" indicates that the
+assembly reads and writes arbitrary undeclared memory locations -- not only the
+memory pointed to by a declared indirect output. Furthermore, the assembly may
+also cause synchronization with other threads, such as via release/acquire
+fences and atomic memory accesses.
 
 Note that clobbering named registers that are also present in output
 constraints is not legal.