[lld] r365139 - [ELF] Allow placing non-string SHF_MERGE sections with different alignments into the same MergeSyntheticSection
Fangrui Song via llvm-commits
llvm-commits at lists.llvm.org
Thu Jul 4 06:33:27 PDT 2019
Author: maskray
Date: Thu Jul 4 06:33:27 2019
New Revision: 365139
URL: http://llvm.org/viewvc/llvm-project?rev=365139&view=rev
Log:
[ELF] Allow placing non-string SHF_MERGE sections with different alignments into the same MergeSyntheticSection
The difference from D63432/r365015 is that this patch does not place
SHF_STRINGS sections with different alignments into the same
MergeSyntheticSection. Doing that would:
(1) create unnecessary padding and thus waste space.
Add a test tail-merge-string-align2.s to check no extra padding is created.
(2) make some input sections unaligned when tail merge (-O2) is enabled.
The alignment of MergeTailAlignment::Builder was out of sync in D63432.
MOVAPS on such unaligned strings can raise SIGSEGV.
This should fix PR42289: the Linux kernel has a use case that input
files have .rodata.cst32 sections with different alignments. The
expectation (and what ld.bfd and gold do) is that in the -r link, there
is only one .rodata.cst32 (SHF_MERGE sections with different alignments
can be combined), but lld currently creates one for each different
alignment.
The current merging strategy:
1) Group SHF_MERGE sections by (name, sh_flags, sh_entsize and
sh_addralign). Merging is performed among a group, even if -O0 is specified.
2) Create one output section for each group. This is a special case in
addInputSec().
This patch changes 1) to:
1) Group SHF_MERGE sections by (name, sh_flags, sh_entsize).
Merging is performed among a group, even if -O0 is specified.
We will thus create just one .rodata.cst32 . This also improves merging
efficiency when sections with the same name but different alignments are
combined.
Reviewed By: peter.smith
Differential Revision: https://reviews.llvm.org/D64200
Added:
lld/trunk/test/ELF/merge-align2.s
lld/trunk/test/ELF/merge-entsize2.s
lld/trunk/test/ELF/tail-merge-string-align2.s
Removed:
lld/trunk/test/ELF/merge-reloc-O0.s
Modified:
lld/trunk/ELF/SyntheticSections.cpp
Modified: lld/trunk/ELF/SyntheticSections.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/SyntheticSections.cpp?rev=365139&r1=365138&r2=365139&view=diff
==============================================================================
--- lld/trunk/ELF/SyntheticSections.cpp (original)
+++ lld/trunk/ELF/SyntheticSections.cpp Thu Jul 4 06:33:27 2019
@@ -2919,6 +2919,8 @@ template <class ELFT> bool VersionNeedSe
void MergeSyntheticSection::addSection(MergeInputSection *MS) {
MS->Parent = this;
Sections.push_back(MS);
+ assert(Alignment == MS->Alignment || !(MS->Flags & SHF_STRINGS));
+ Alignment = std::max(Alignment, MS->Alignment);
}
MergeTailSection::MergeTailSection(StringRef Name, uint32_t Type,
@@ -3062,8 +3064,11 @@ void elf::mergeSections() {
//
// Using Entsize in here also allows us to propagate it to the synthetic
// section.
+ //
+ // SHF_STRINGS section with different alignments should not be merged.
return Sec->Name == OutsecName && Sec->Flags == MS->Flags &&
- Sec->Entsize == MS->Entsize && Sec->Alignment == MS->Alignment;
+ Sec->Entsize == MS->Entsize &&
+ (Sec->Alignment == MS->Alignment || !(Sec->Flags & SHF_STRINGS));
});
if (I == MergeSections.end()) {
MergeSyntheticSection *Syn =
Added: lld/trunk/test/ELF/merge-align2.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/merge-align2.s?rev=365139&view=auto
==============================================================================
--- lld/trunk/test/ELF/merge-align2.s (added)
+++ lld/trunk/test/ELF/merge-align2.s Thu Jul 4 06:33:27 2019
@@ -0,0 +1,35 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-readelf -S %t | FileCheck --check-prefix=SEC %s
+# RUN: llvm-readelf -x .cst8 %t | FileCheck %s
+
+# RUN: ld.lld -O0 -r %t.o -o %t1.o
+# RUN: llvm-readelf -S %t1.o | FileCheck --check-prefix=SEC %s
+# RUN: llvm-readelf -x .cst8 %t1.o | FileCheck %s
+
+## Check that if we have SHF_MERGE sections with the same name, flags and
+## entsize, but different alignments, we combine them with the maximum input
+## alignment as the output alignment.
+
+# SEC: Name Type {{.*}} Size ES Flg Lk Inf Al
+# SEC: .cst8 PROGBITS {{.*}} 000018 08 AM 0 0 8
+
+# CHECK: 0x{{[0-9a-f]+}} 02000000 00000000 01000000 00000000
+# CHECK-NEXT: 0x{{[0-9a-f]+}} 03000000 00000000
+
+.section .cst8,"aM", at progbits,8,unique,0
+.align 4
+.quad 1
+.quad 1
+
+.section .cst8,"aM", at progbits,8,unique,1
+.align 4
+.quad 1
+.quad 2
+
+.section .cst8,"aM", at progbits,8,unique,2
+.align 8
+.quad 1
+.quad 3
Added: lld/trunk/test/ELF/merge-entsize2.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/merge-entsize2.s?rev=365139&view=auto
==============================================================================
--- lld/trunk/test/ELF/merge-entsize2.s (added)
+++ lld/trunk/test/ELF/merge-entsize2.s Thu Jul 4 06:33:27 2019
@@ -0,0 +1,49 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-readelf -S %t | FileCheck --check-prefix=SEC %s
+# RUN: llvm-readelf -x .cst %t | FileCheck --check-prefix=HEX %s
+
+# RUN: ld.lld -O0 -r %t.o -o %t1.o
+# RUN: llvm-readelf -S %t1.o | FileCheck --check-prefix=SEC-R %s
+# RUN: llvm-readelf -x .cst %t1.o | FileCheck --check-prefix=HEX-R %s
+
+## Check that SHF_MERGE sections with the same name, sh_flags and sh_entsize
+## are grouped together and can be merged within the group.
+
+## .cst 0 and .cst 1 are merged (sh_entsize=4). The result and .cst 2 and
+## combined (sh_entsize=8). The output sh_entsize is 0.
+# SEC: Name Type {{.*}} Size ES Flg Lk Inf Al
+# SEC: .cst PROGBITS {{.*}} 000020 00 AM 0 0 8
+
+## .cst 0 and .cst 1 are merged, but emitted as a separate output section.
+# SEC-R: .cst PROGBITS {{.*}} 00000c 04 AM 0 0 4
+# SEC-R: .cst PROGBITS {{.*}} 000010 08 AM 0 0 8
+
+# HEX: Hex dump of section '.cst':
+# HEX-NEXT: 0x{{[0-9a-f]+}} 01000000 00000000 02000000 00000000
+# HEX-NEXT: 0x{{[0-9a-f]+}} 01000000 00000000 03000000 00000000
+
+# HEX-R: Hex dump of section '.cst':
+# HEX-R-NEXT: 0x00000000 01000000 00000000 02000000
+# HEX-R-EMPTY:
+# HEX-R-NEXT: Hex dump of section '.cst':
+# HEX-R-NEXT: 0x00000000 01000000 00000000 03000000 00000000
+
+.section .cst,"aM", at progbits,4,unique,0
+.align 2
+.long 1
+.long 0
+.long 2
+
+.section .cst,"aM", at progbits,4,unique,1
+.align 4
+.long 1
+.long 0
+.long 2
+
+.section .cst,"aM", at progbits,8,unique,2
+.align 8
+.quad 1
+.quad 3
Removed: lld/trunk/test/ELF/merge-reloc-O0.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/merge-reloc-O0.s?rev=365138&view=auto
==============================================================================
--- lld/trunk/test/ELF/merge-reloc-O0.s (original)
+++ lld/trunk/test/ELF/merge-reloc-O0.s (removed)
@@ -1,48 +0,0 @@
-# REQUIRES: x86
-# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
-# RUN: ld.lld %t.o -r -o %t2.o -O0
-# RUN: llvm-readobj -S --section-data %t2.o | FileCheck %s
-
-# We combine just the sections with the same name and sh_entsize.
-
-# CHECK: Name: .foo
-# CHECK-NEXT: Type: SHT_PROGBITS
-# CHECK-NEXT: Flags [
-# CHECK-NEXT: SHF_ALLOC
-# CHECK-NEXT: SHF_MERGE
-# CHECK-NEXT: ]
-# CHECK-NEXT: Address:
-# CHECK-NEXT: Offset:
-# CHECK-NEXT: Size: 16
-# CHECK-NEXT: Link:
-# CHECK-NEXT: Info:
-# CHECK-NEXT: AddressAlignment: 1
-# CHECK-NEXT: EntrySize: 8
-# CHECK-NEXT: SectionData (
-# CHECK-NEXT: 0000: 41000000 00000000 42000000 00000000
-# CHECK-NEXT: )
-
-# CHECK: Name: .foo
-# CHECK-NEXT: Type: SHT_PROGBITS
-# CHECK-NEXT: Flags [
-# CHECK-NEXT: SHF_ALLOC
-# CHECK-NEXT: SHF_MERGE
-# CHECK-NEXT: ]
-# CHECK-NEXT: Address:
-# CHECK-NEXT: Offset:
-# CHECK-NEXT: Size: 8
-# CHECK-NEXT: Link:
-# CHECK-NEXT: Info:
-# CHECK-NEXT: AddressAlignment: 1
-# CHECK-NEXT: EntrySize: 4
-# CHECK-NEXT: SectionData (
-# CHECK-NEXT: 0000: 41000000 42000000
-# CHECK-NEXT: )
-
- .section .foo, "aM", at progbits,8,unique,0
- .quad 0x41
- .section .foo, "aM", at progbits,8,unique,1
- .quad 0x42
- .section .foo, "aM", at progbits,4,unique,2
- .long 0x41
- .long 0x42
Added: lld/trunk/test/ELF/tail-merge-string-align2.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/tail-merge-string-align2.s?rev=365139&view=auto
==============================================================================
--- lld/trunk/test/ELF/tail-merge-string-align2.s (added)
+++ lld/trunk/test/ELF/tail-merge-string-align2.s Thu Jul 4 06:33:27 2019
@@ -0,0 +1,25 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-readelf -S %t | FileCheck --check-prefix=SEC %s
+# RUN: llvm-readelf -x .rodata %t | FileCheck %s
+
+# SEC: Name Type {{.*}} Size ES Flg Lk Inf Al
+# SEC: .rodata PROGBITS {{.*}} 000006 01 AMS 0 0 8
+
+## Check there is no extra padding.
+
+# CHECK: a.b.c.
+
+.section .rodata.str1.8,"aMS", at progbits,1
+.align 8
+.asciz "a"
+
+.section .rodata.str1.2,"aMS", at progbits,1
+.align 2
+.asciz "b"
+
+.section .rodata.str1.1,"aMS", at progbits,1
+.align 1
+.asciz "c"
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