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[LangRef] Clarify support for multiple metadata attachments with same id 

As discussed on D105251, currently the compiler does not support
multiple metadata attachments on instructions having the same
identifier, whereas it does for global objects. Note this in the
Language Reference manual for clarity.

See D105251 for discussions of history behind this divergence, and the
complexities and possible approaches of adding this support to
instructions in the future.

Differential Revision: https://reviews.llvm.org/D106304
This commit is contained in:
Teresa Johnson 2021-07-19 12:35:45 -07:00
parent 51e62e56f7
commit 54c8902f02
1 changed files with 8 additions and 2 deletions
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10
llvm/docs/LangRef.rst
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@ -5035,8 +5035,8 @@ occurs on.
Metadata
========
LLVM IR allows metadata to be attached to instructions in the program
that can convey extra information about the code to the optimizers and
LLVM IR allows metadata to be attached to instructions and global objects in the
program that can convey extra information about the code to the optimizers and
code generator. One example application of metadata is source-level
debug information. There are two metadata primitives: strings and nodes.
@ -5096,6 +5096,9 @@ to the ``add`` instruction using the ``!dbg`` identifier:
%indvar.next = add i64 %indvar, 1, !dbg !21
Instructions may not have multiple metadata attachments with the same
identifier.
Metadata can also be attached to a function or a global variable. Here metadata
``!22`` is attached to the ``f1`` and ``f2`` functions, and the globals ``g1``
and ``g2`` using the ``!dbg`` identifier:
@ -5110,6 +5113,9 @@ and ``g2`` using the ``!dbg`` identifier:
@g1 = global i32 0, !dbg !22
@g2 = external global i32, !dbg !22
Unlike instructions, global objects (functions and global variables) may have
multiple metadata attachments with the same identifier.
A transformation is required to drop any metadata attachment that it does not
know or know it can't preserve. Currently there is an exception for metadata
attachment to globals for ``!type`` and ``!absolute_symbol`` which can't be