584cb67d2d
IR symbol table does not parse inline asm. A symbol only referenced by inline
asm is not in the IR symbol table, so LTO does not know that the definition (in
another translation unit) is referenced and may internalize it, even if that
definition has `__attribute__((used))` (which lowers to `llvm.compiler.used` on
ELF targets since D97446).
```
// cabac.c
__attribute__((used)) const uint8_t ff_h264_cabac_tables[...] = {...};
// h264_cabac.c
asm("lea ff_h264_cabac_tables(%rip), %0" : ...);
```
`__attribute__((used))` is the recommended way to tell the compiler there may
be inline asm references, so the usage is perfectly fine. This patch
conservatively sets the `FB_used` bit on `llvm.compiler.used` symbols to work
around the IR symbol table limitation. Note: before D97446, Clang never emitted
symbols in the `llvm.compiler.used` list, so this change does not punish any
Clang emitted global object.
Without the patch, `ff_h264_cabac_tables` may be assigned to a non-external
partition and get internalized. Then we will get a linker error because the
`cabac.c` definition is not exposed.
Differential Revision: https://reviews.llvm.org/D97755
30 lines
1.1 KiB
C
30 lines
1.1 KiB
C
// REQUIRES: x86-registered-target
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// RUN: split-file %s %t
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// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -flto=thin -emit-llvm-bc %t/a.c -o %t/a.bc
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// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -flto=thin -emit-llvm-bc %t/b.c -o %t/b.bc
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// RUN: llvm-nm %t/a.bc | FileCheck %s --check-prefix=NM
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// RUN: llvm-lto2 run %t/a.bc %t/b.bc -o %t/out -save-temps -r=%t/a.bc,ref,plx -r=%t/b.bc,ff_h264_cabac_tables,pl
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// RUN: llvm-dis < %t/out.2.2.internalize.bc | FileCheck %s
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//--- a.c
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/// IR symtab does not track inline asm symbols, so we don't know
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/// ff_h264_cabac_tables is undefined.
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// NM-NOT: {{.}}
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// NM: ---------------- T ref
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// NM-NOT: {{.}}
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const char *ref() {
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const char *ret;
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asm("lea ff_h264_cabac_tables(%%rip), %0" : "=r"(ret));
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return ret;
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}
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//--- b.c
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/// ff_h264_cabac_tables has __attribute__((used)) in the source code, which means
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/// its definition must be retained because there can be references the compiler
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/// cannot see (inline asm reference). Test we don't internalize it.
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// CHECK: @ff_h264_cabac_tables = dso_local constant [1 x i8] c"\09"
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__attribute__((used))
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const char ff_h264_cabac_tables[1] = "\x09";
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