06eee734c1
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(&map1, cb, &cb_data, 0);
} else {
bpf_for_each_map_elem(&map2, cb, &cb_data, 0);
}
Is converted to bytecode equivalent to:
if (data_end - data > 1024)
tmp = &map1;
else
tmp = &map2;
bpf_for_each_map_elem(tmp, cb, &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-f374-1e67-69d6-76dd9c8318a4@meta.com/
Differential Revision: https://reviews.llvm.org/D152986
93 lines
5.1 KiB
C
93 lines
5.1 KiB
C
// RUN: %clang_cc1 -fsyntax-only -verify %s
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#if !__has_extension(statement_attributes_with_gnu_syntax)
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#error "We should have statement attributes with GNU syntax support"
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#endif
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void foo(int i) {
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__attribute__((unknown_attribute)); // expected-warning {{unknown attribute 'unknown_attribute' ignored}}
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__attribute__(()) {}
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__attribute__(()) if (0) {}
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__attribute__(()) for (;;);
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__attribute__(()) do {
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__attribute__(()) continue;
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}
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while (0)
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;
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__attribute__(()) while (0);
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__attribute__(()) switch (i) {
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__attribute__(()) case 0 :
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__attribute__(()) default :
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__attribute__(()) break;
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}
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__attribute__(()) goto here;
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__attribute__(()) here :
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__attribute__(()) return;
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__attribute__((noreturn)) {} // expected-error {{'noreturn' attribute cannot be applied to a statement}}
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__attribute__((noreturn)) if (0) {} // expected-error {{'noreturn' attribute cannot be applied to a statement}}
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__attribute__((noreturn)) for (;;); // expected-error {{'noreturn' attribute cannot be applied to a statement}}
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__attribute__((noreturn)) do { // expected-error {{'noreturn' attribute cannot be applied to a statement}}
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__attribute__((unavailable)) continue; // expected-error {{'unavailable' attribute cannot be applied to a statement}}
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}
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while (0)
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;
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__attribute__((unknown_attribute)) while (0); // expected-warning {{unknown attribute 'unknown_attribute' ignored}}
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__attribute__((unused)) switch (i) { // expected-error {{'unused' attribute cannot be applied to a statement}}
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__attribute__((uuid)) case 0: // expected-warning {{unknown attribute 'uuid' ignored}}
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__attribute__((visibility(""))) default: // expected-error {{'visibility' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) break; // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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}
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__attribute__((fastcall)) goto there; // expected-error {{'fastcall' attribute cannot be applied to a statement}}
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__attribute__((noinline)) there : // expected-warning {{'noinline' attribute only applies to functions and statements}}
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__attribute__((weakref)) return; // expected-error {{'weakref' attribute only applies to variables and functions}}
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__attribute__((carries_dependency)); // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) {} // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) if (0) {} // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) for (;;); // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) do { // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) continue; // expected-error {{'carries_dependency' attribute cannot be applied to a statement}} ignored}}
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}
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while (0)
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;
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__attribute__((carries_dependency)) while (0); // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) switch (i) { // expected-error {{'carries_dependency' attribute cannot be applied to a statement}} ignored}}
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__attribute__((carries_dependency)) case 0: // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) default: // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) break; // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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}
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__attribute__((carries_dependency)) goto here; // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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__attribute__((carries_dependency)) return; // expected-error {{'carries_dependency' attribute cannot be applied to a statement}}
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}
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void bar(void);
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void foobar(void) {
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__attribute__((nomerge)) bar();
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__attribute__(()) bar(); // expected-error {{expected identifier or '('}}
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__attribute__((unused, nomerge)) bar(); // expected-error {{expected identifier or '('}}
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__attribute__((nomerge, unused)) bar(); // expected-error {{expected identifier or '('}}
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__attribute__((nomerge(1, 2))) bar(); // expected-error {{'nomerge' attribute takes no arguments}}
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int x;
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__attribute__((nomerge)) x = 10; // expected-warning {{'nomerge' attribute is ignored because there exists no call expression inside the statement}}
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__attribute__((nomerge)) label : bar(); // expected-error {{'nomerge' attribute only applies to functions, statements and variables}}
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}
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int f(void);
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__attribute__((nomerge)) static int (*fptr)(void);
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__attribute__((nomerge)) static int i; // expected-warning {{'nomerge' attribute is ignored because 'i' is not a function pointer}}
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struct buz {} __attribute__((nomerge)); // expected-error {{'nomerge' attribute only applies to functions, statements and variables}}
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