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BPF: remove default .extern data section 

Currently, for any extern variable, if it doesn't have
section attribution, it will be put into a default ".extern"
btf DataSec. The initial design is to put every extern
variable in a DataSec so libbpf can use it.

But later on, libbpf actually requires extern variables
to put into special sections, e.g., ".kconfig", ".ksyms", etc.
so they can be used properly based on section name.

Andrii mentioned since ".extern" variables are
not actually used, it makes sense to remove it from
the compiler so libbpf does not need to deal with it,
esp. for static linking. The BTF for these extern variables
is still generated.

With this patch, I tested kernel selftests/bpf and all tests
passed. Indeed, removing ".extern" DataSec seems having no
impact.

Differential Revision: https://reviews.llvm.org/D100392
This commit is contained in:
Yonghong Song 2021-04-13 09:15:21 -07:00
parent b351590bae
commit a285bdb56f
3 changed files with 9 additions and 27 deletions
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8
llvm/lib/Target/BPF/BTFDebug.cpp
View File

@ -1146,10 +1146,6 @@ void BTFDebug::processGlobals(bool ProcessingMapDef) {
SecName = ".rodata";
else
SecName = Global.getInitializer()->isZeroValue() ? ".bss" : ".data";
} else {
// extern variables without explicit section,
// put them into ".extern" section.
SecName = ".extern";
}
if (ProcessingMapDef != SecName.startswith(".maps"))
@ -1213,7 +1209,9 @@ void BTFDebug::processGlobals(bool ProcessingMapDef) {
std::make_unique<BTFKindVar>(Global.getName(), GVTypeId, GVarInfo);
uint32_t VarId = addType(std::move(VarEntry));
assert(!SecName.empty());
// An empty SecName means an extern variable without section attribute.
if (SecName.empty())
continue;
// Find or create a DataSec
if (DataSecEntries.find(std::string(SecName)) == DataSecEntries.end()) {

14
llvm/test/CodeGen/BPF/BTF/extern-var-struct-weak.ll
View File

@ -24,9 +24,9 @@ entry:
; CHECK-NEXT: .byte 0
; CHECK-NEXT: .long 24
; CHECK-NEXT: .long 0
; CHECK-NEXT: .long 116
; CHECK-NEXT: .long 116
; CHECK-NEXT: .long 81
; CHECK-NEXT: .long 92
; CHECK-NEXT: .long 92
; CHECK-NEXT: .long 73
; CHECK-NEXT: .long 0 # BTF_KIND_FUNC_PROTO(id = 1)
; CHECK-NEXT: .long 218103808 # 0xd000000
; CHECK-NEXT: .long 2
@ -50,12 +50,6 @@ entry:
; CHECK-NEXT: .long 234881024 # 0xe000000
; CHECK-NEXT: .long 4
; CHECK-NEXT: .long 2
; CHECK-NEXT: .long 73 # BTF_KIND_DATASEC(id = 7)
; CHECK-NEXT: .long 251658241 # 0xf000001
; CHECK-NEXT: .long 0
; CHECK-NEXT: .long 6
; CHECK-NEXT: .long global
; CHECK-NEXT: .long 4
; CHECK-NEXT: .byte 0 # string offset=0
; CHECK-NEXT: .ascii "int" # string offset=1
; CHECK-NEXT: .byte 0
@ -73,8 +67,6 @@ entry:
; CHECK-NEXT: .byte 0
; CHECK-NEXT: .ascii "global" # string offset=66
; CHECK-NEXT: .byte 0
; CHECK-NEXT: .ascii ".extern" # string offset=73
; CHECK-NEXT: .byte 0
attributes #0 = { norecurse nounwind readonly "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }

14
llvm/test/CodeGen/BPF/BTF/extern-var-struct.ll
View File

@ -25,9 +25,9 @@ entry:
; CHECK-NEXT: .byte 0
; CHECK-NEXT: .long 24
; CHECK-NEXT: .long 0
; CHECK-NEXT: .long 116
; CHECK-NEXT: .long 116
; CHECK-NEXT: .long 81
; CHECK-NEXT: .long 92
; CHECK-NEXT: .long 92
; CHECK-NEXT: .long 73
; CHECK-NEXT: .long 0 # BTF_KIND_FUNC_PROTO(id = 1)
; CHECK-NEXT: .long 218103808 # 0xd000000
; CHECK-NEXT: .long 2
@ -51,12 +51,6 @@ entry:
; CHECK-NEXT: .long 234881024 # 0xe000000
; CHECK-NEXT: .long 4
; CHECK-NEXT: .long 2
; CHECK-NEXT: .long 73 # BTF_KIND_DATASEC(id = 7)
; CHECK-NEXT: .long 251658241 # 0xf000001
; CHECK-NEXT: .long 0
; CHECK-NEXT: .long 6
; CHECK-NEXT: .long global
; CHECK-NEXT: .long 4
; CHECK-NEXT: .byte 0 # string offset=0
; CHECK-NEXT: .ascii "int" # string offset=1
; CHECK-NEXT: .byte 0
@ -74,8 +68,6 @@ entry:
; CHECK-NEXT: .byte 0
; CHECK-NEXT: .ascii "global" # string offset=66
; CHECK-NEXT: .byte 0
; CHECK-NEXT: .ascii ".extern" # string offset=73
; CHECK-NEXT: .byte 0
attributes #0 = { norecurse nounwind readonly "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }