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
21 lines
626 B
C++
21 lines
626 B
C++
// RUN: %clang_cc1 -ast-dump %s 2>&1 | FileCheck %s
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[[clang::nomerge]] void func();
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[[clang::nomerge]] void func();
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void func();
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[[clang::nomerge]] void func() {}
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[[clang::nomerge]] void (*var)(void);
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// CHECK: FunctionDecl {{.*}} func 'void ()'
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// CHECK-NEXT: NoMergeAttr
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// CHECK-NEXT: FunctionDecl {{.*}} func 'void ()'
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// CHECK-NEXT: NoMergeAttr
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// CHECK-NEXT: FunctionDecl {{.*}} func 'void ()'
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// CHECK-NEXT: NoMergeAttr {{.*}} Inherited
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// CHECK-NEXT: FunctionDecl {{.*}} func 'void ()'
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// CHECK-NEXT: CompoundStmt
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// CHECK-NEXT: NoMergeAttr
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// CHECK-NEXT: VarDecl {{.*}} var 'void (*)()'
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// CHECK-NEXT: NoMergeAttr
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