[llvm] clarify semantics of masked vector load/store (PR #82469)

[Ralf Jung via llvm-commits]

https://github.com/RalfJung updated https://github.com/llvm/llvm-project/pull/82469

>From acf542299fe436284966930c17db9481e0bc2c27 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post at ralfj.de>
Date: Wed, 21 Feb 2024 08:42:04 +0100
Subject: [PATCH] clarify semantics of masked vector load/store

---
 llvm/docs/LangRef.rst | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index fd2e3aacd0169c..6abeaa1bd4d7da 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -23751,7 +23751,9 @@ Semantics:
 """"""""""
 
 The '``llvm.masked.load``' intrinsic is designed for conditional reading of selected vector elements in a single IR operation. It is useful for targets that support vector masked loads and allows vectorizing predicated basic blocks on these targets. Other targets may support this intrinsic differently, for example by lowering it into a sequence of branches that guard scalar load operations.
-The result of this operation is equivalent to a regular vector load instruction followed by a 'select' between the loaded and the passthru values, predicated on the same mask. However, using this intrinsic prevents exceptions on memory access to masked-off lanes.
+The result of this operation is equivalent to a regular vector load instruction followed by a 'select' between the loaded and the passthru values, predicated on the same mask, except that the masked-off lanes are not accessed.
+Only the masked-on lanes of the vector need to be inbounds of an allocation (but all these lanes need to be inbounds of the same allocation).
+In particular, using this intrinsic prevents exceptions on memory access to masked-off lanes.
 
 
 ::
@@ -23793,7 +23795,9 @@ Semantics:
 """"""""""
 
 The '``llvm.masked.store``' intrinsics is designed for conditional writing of selected vector elements in a single IR operation. It is useful for targets that support vector masked store and allows vectorizing predicated basic blocks on these targets. Other targets may support this intrinsic differently, for example by lowering it into a sequence of branches that guard scalar store operations.
-The result of this operation is equivalent to a load-modify-store sequence. However, using this intrinsic prevents exceptions and data races on memory access to masked-off lanes.
+The result of this operation is equivalent to a load-modify-store sequence, except that the masked-off lanes are not accessed.
+Only the masked-on lanes of the vector need to be inbounds of an allocation (but all these lanes need to be inbounds of the same allocation).
+In particular, using this intrinsic prevents exceptions and data races on memory access to masked-off lanes.
 
 ::