Author: Eduard Zingerman
Date: 2023-06-27T01:15:45+03:00
New Revision: 06eee734c1ea9de754c1e7e8c79c0897b9e01350
URL:
https://github.com/llvm/llvm-project/commit/06eee734c1ea9de754c1e7e8c79c0897b9e01350
DIFF:
https://github.com/llvm/llvm-project/commit/06eee734c1ea9de754c1e7e8c79c0897b9e01350.diff
LOG: [clang] Allow 'nomerge' attribute for function pointers
Allow specifying 'nomerge' attribute for function pointers,
e.g. like in the following C code:
extern void (*foo)(void) __attribute__((nomerge));
void bar(long i) {
if (i)
foo();
else
foo();
}
With the goal to attach 'nomerge' to both calls done through 'foo':
@foo = external local_unnamed_addr global ptr, align 8
define dso_local void @bar(i64 noundef %i) local_unnamed_addr #0 {
; ...
%0 = load ptr, ptr @foo, align 8, !tbaa !5
; ...
if.then:
tail call void %0() #1
br label %if.end
if.else:
tail call void %0() #1
br label %if.end
if.end:
ret void
}
; ...
attributes #1 = { nomerge ... }
Report a warning in case if 'nomerge' is specified for a variable that
is not a function pointer, e.g.:
t.c:2:22: warning: 'nomerge' attribute is ignored because 'j' is not a
function pointer [-Wignored-attributes]
2 | int j __attribute__((nomerge));
| ^
The intended use-case is for BPF backend.
BPF provides a sort of "standard library" functions that are called
helpers. BPF also verifies usage of these helpers before program
execution. Because of limitations of verification / runtime model it
is important to keep calls to some of such helpers from merging.
An example could be found by the link [1], there input C code:
if (data_end - data > 1024) {
bpf_for_each_map_elem(, cb, _data, 0);
} else {
bpf_for_each_map_elem(, cb, _data, 0);
}
Is converted to bytecode equivalent to:
if (data_end - data > 1024)
tmp =
else
tmp =
bpf_for_each_map_elem(tmp, cb, _data, 0);
However, BPF verification/runtime requires to use the same map address
for each particular `bpf_for_each_map_elem()` call.
The 'nomerge' attribute is a perfect match for this situation, but
unfortunately BPF helpers are declared as pointers to functions:
static long (*bpf_for_each_map_elem)(void *map, ...) = (void *) 164;
Hence, this commit, allowing to use 'nomerge' for function pointers.
[1] https://lore.kernel.org/bpf/03bdf90f-f374-1e67-69d6-76dd9c831...@meta.com/
Differential Revision: https://reviews.llvm.org/D152986
Added:
Modified:
clang/include/clang/Basic/Attr.td
clang/include/clang/Basic/AttrDocs.td
clang/include/clang/Basic/DiagnosticSemaKinds.td
clang/lib/CodeGen/CGCall.cpp
clang/lib/Sema/SemaDeclAttr.cpp
clang/test/CodeGen/attr-nomerge.cpp
clang/test/Misc/pragma-attribute-supported-attributes-list.test
clang/test/Parser/stmt-attributes.c
clang/test/Parser/stmt-attributes.cpp
clang/test/Parser/stmt-attributes.m
clang/test/Sema/attr-nomerge-ast.cpp
clang/test/Sema/attr-nomerge.cpp
Removed:
diff --git a/clang/include/clang/Basic/Attr.td
b/clang/include/clang/Basic/Attr.td
index b9e701641fdf1..d5204b2869667 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -1482,9 +1482,8 @@ def : MutualExclusions<[Likely, Unlikely]>;
def NoMerge : DeclOrStmtAttr {
let Spellings = [Clang<"nomerge">];
let Documentation = [NoMergeDocs];
- let Subjects = SubjectList<[Function, Stmt], ErrorDiag,
- "functions and statements">;
- let SimpleHandler = 1;
+ let Subjects = SubjectList<[Function, Stmt, Var], ErrorDiag,
+ "functions, statements and variables">;
}
def MustTail : StmtAttr {
diff --git a/clang/include/clang/Basic/AttrDocs.td
b/clang/include/clang/Basic/AttrDocs.td
index 5178ca43b9710..2c950231255d7 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -549,7 +549,31 @@ over the tradeoff between code size and debug information
precision.
``nomerge`` attribute can also be used as function attribute to prevent all
calls to the specified function from merging. It has no effect on indirect
-calls.
+calls to such functions. For example:
+
+.. code-block:: c++
+
+ [[clang::nomerge]] void foo(int) {}
+
+ void bar(int x) {
+auto *ptr = foo;
+if (x) foo(1); else foo(2); // will not be merged
+if (x) ptr(1); else ptr(2); // indirect call, can be merged
+ }
+
+``nomerge`` attribute can also be used for pointers to functions to
+prevent calls through such pointer from merging. In such case the
+effect applies only to a specific function pointer. For example:
+
+.. code-block:: c++
+
+ [[clang::nomerge]] void (*foo)(int);
+
+ void bar(int x) {
+auto *ptr =
