[llvm] 1e55cf7 - [LangRef] Define mustprogress attribute
Atmn Patel via llvm-commits
llvm-commits at lists.llvm.org
Mon Oct 19 10:35:55 PDT 2020
Author: Atmn Patel
Date: 2020-10-19T13:34:27-04:00
New Revision: 1e55cf77f389c5d13a776228c6d7128dad638b03
URL: https://github.com/llvm/llvm-project/commit/1e55cf77f389c5d13a776228c6d7128dad638b03
DIFF: https://github.com/llvm/llvm-project/commit/1e55cf77f389c5d13a776228c6d7128dad638b03.diff
LOG: [LangRef] Define mustprogress attribute
LLVM IR currently assumes some form of forward progress. This form is
not explicitly defined anywhere, and is the cause of miscompilations
in most languages that are not C++11 or later. This implicit forward progress
guarantee can not be opted out of on a function level nor on a loop
level. Languages such as C (C11 and later), C++ (pre-C++11), and Rust
have different forward progress requirements and this needs to be
evident in the IR.
Specifically, C11 and onwards (6.8.5, Paragraph 6) states that "An
iteration statement whose controlling expression is not a constant
expression, that performs no input/output operations, does not access
volatile objects, and performs no synchronization or atomic operations
in its body, controlling expression, or (in the case of for statement)
its expression-3, may be assumed by the implementation to terminate."
C++11 and onwards does not have this assumption, and instead assumes
that every thread must make progress as defined in [intro.progress] when
it comes to scheduling.
This was initially brought up in [0] as a bug, a solution was presented
in [1] which is the current workaround, and the predecessor to this
change was [2].
After defining a notion of forward progress for IR, there are two
options to address this:
1) Set the default to assuming Forward Progress and provide an opt-out for functions and an opt-in for loops.
2) Set the default to not assuming Forward Progress and provide an opt-in for functions, and an opt-in for loops.
Option 2) has been selected because only C++11 and onwards have a
forward progress requirement and it makes sense for them to opt-into it
via the defined `mustprogress` function attribute. The `mustprogress`
function attribute indicates that the function is required to make
forward progress as defined. This is sharply in contrast to the status
quo where this is implicitly assumed. In addition, `willreturn` implies `mustprogress`.
The background for why this definition was chosen is in [3] and for why
the option was chosen is in [4] and the corresponding thread(s). The implementation is in D85393, the
clang patch is in D86841, the LoopDeletion patch is in D86844, the
Inliner patches are in D87180 and D87262, and there will be more
incoming.
[0] https://bugs.llvm.org/show_bug.cgi?id=965#c25
[1] https://lists.llvm.org/pipermail/llvm-dev/2017-October/118558.html
[2] https://reviews.llvm.org/D65718
[3] https://lists.llvm.org/pipermail/llvm-dev/2020-September/144919.html
[4] https://lists.llvm.org/pipermail/llvm-dev/2020-September/145023.html
Reviewed By: jdoerfert, efriedma, nikic
Differential Revision: https://reviews.llvm.org/D86233
Added:
Modified:
llvm/docs/LangRef.rst
llvm/utils/emacs/llvm-mode.el
llvm/utils/kate/llvm.xml
llvm/utils/llvm.grm
llvm/utils/vim/syntax/llvm.vim
llvm/utils/vscode/llvm/syntaxes/ll.tmLanguage.yaml
Removed:
################################################################################
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index b36c544f9de1..ede5c5047570 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1958,6 +1958,18 @@ example:
the function. The instrumentation checks that the return address for the
function has not changed between the function prolog and epilog. It is
currently x86_64-specific.
+``mustprogress``
+ This attribute indicates that the function is required to return, unwind,
+ or interact with the environment in an observable way e.g. via a volatile
+ memory access, I/O, or other synchronization. The ``mustprogress``
+ attribute is intended to model the requirements of the first section of
+ [intro.progress] of the C++ Standard. As a consequence, a loop in a
+ function with the `mustprogress` attribute can be assumed to terminate if
+ it does not interact with the environment in an observable way, and
+ terminating loops without side-effects can be removed. If a `mustprogress`
+ function does not satisfy this contract, the behavior is undefined. This
+ attribute does not apply transitively to callees, but does apply to call
+ sites within the function. Note that `willreturn` implies `mustprogress`.
Call Site Attributes
----------------------
diff --git a/llvm/utils/emacs/llvm-mode.el b/llvm/utils/emacs/llvm-mode.el
index 6852899e2a35..77cc6a7bebdc 100644
--- a/llvm/utils/emacs/llvm-mode.el
+++ b/llvm/utils/emacs/llvm-mode.el
@@ -23,7 +23,7 @@
;; Attributes
`(,(regexp-opt
'("alwaysinline" "argmemonly" "builtin" "cold" "convergent" "inaccessiblememonly"
- "inaccessiblemem_or_argmemonly" "inlinehint" "jumptable" "minsize" "naked" "nobuiltin"
+ "inaccessiblemem_or_argmemonly" "inlinehint" "jumptable" "minsize" "mustprogress" "naked" "nobuiltin"
"noduplicate" "nofree" "noimplicitfloat" "noinline" "nonlazybind" "noredzone" "noreturn"
"norecurse" "noundef" "nounwind" "optnone" "optsize" "readnone" "readonly" "returns_twice"
"speculatable" "ssp" "sspreq" "sspstrong" "safestack" "sanitize_address" "sanitize_hwaddress" "sanitize_memtag"
diff --git a/llvm/utils/kate/llvm.xml b/llvm/utils/kate/llvm.xml
index 6e08af54eaa4..5132aaf6fba5 100644
--- a/llvm/utils/kate/llvm.xml
+++ b/llvm/utils/kate/llvm.xml
@@ -81,6 +81,7 @@
<item> argmemonly </item>
<item> convergent </item>
<item> inlinehint </item>
+ <item> mustprogress </item>
<item> naked </item>
<item> nobuiltin </item>
<item> noimplicitfloat </item>
diff --git a/llvm/utils/llvm.grm b/llvm/utils/llvm.grm
index 92a4053173d6..398c04d9b6da 100644
--- a/llvm/utils/llvm.grm
+++ b/llvm/utils/llvm.grm
@@ -175,6 +175,7 @@ FuncAttr ::= noreturn
| sanitize_address
| sanitize_thread
| sanitize_memory
+ | mustprogress
;
OptFuncAttrs ::= + _ | OptFuncAttrs FuncAttr ;
diff --git a/llvm/utils/vim/syntax/llvm.vim b/llvm/utils/vim/syntax/llvm.vim
index 68c41e79d657..0914f6de3829 100644
--- a/llvm/utils/vim/syntax/llvm.vim
+++ b/llvm/utils/vim/syntax/llvm.vim
@@ -107,6 +107,7 @@ syn keyword llvmKeyword
\ module
\ monotonic
\ msp430_intrcc
+ \ mustprogress
\ musttail
\ naked
\ nest
diff --git a/llvm/utils/vscode/llvm/syntaxes/ll.tmLanguage.yaml b/llvm/utils/vscode/llvm/syntaxes/ll.tmLanguage.yaml
index 60c5b13d2741..49c2dacd4c9e 100644
--- a/llvm/utils/vscode/llvm/syntaxes/ll.tmLanguage.yaml
+++ b/llvm/utils/vscode/llvm/syntaxes/ll.tmLanguage.yaml
@@ -204,6 +204,7 @@ patterns:
\\bmodule\\b|\
\\bmonotonic\\b|\
\\bmsp430_intrcc\\b|\
+ \\bmustprogress\\b|\
\\bmusttail\\b|\
\\bnaked\\b|\
\\bnest\\b|\
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