[lld] 8aea109 - [ELF] x86-64: place .lrodata, .lbss, and .ldata away from code sections
Fangrui Song via llvm-commits
llvm-commits at lists.llvm.org
Thu May 25 07:35:44 PDT 2023
Author: Fangrui Song
Date: 2023-05-25T07:35:38-07:00
New Revision: 8aea10950463f915781cca6affde963e4565db2f
URL: https://github.com/llvm/llvm-project/commit/8aea10950463f915781cca6affde963e4565db2f
DIFF: https://github.com/llvm/llvm-project/commit/8aea10950463f915781cca6affde963e4565db2f.diff
LOG: [ELF] x86-64: place .lrodata, .lbss, and .ldata away from code sections
The x86-64 medium code model utilizes large data sections, namely .lrodata,
.lbss, and .ldata (along with some variants of .ldata). There is a proposal to
extend the use of large data sections to the large code model as well[1].
This patch aims to place large data sections away from code sections in order to
alleviate relocation overflow pressure caused by code sections referencing
regular data sections.
```
.lrodata
.rodata
.text # if --ro-segment, MAXPAGESIZE alignment
RELRO # MAXPAGESIZE alignment
.data # MAXPAGESIZE alignment
.bss
.ldata # MAXPAGESIZE alignment
.lbss
```
In comparison to GNU ld, which places .lbss, .lrodata, and .ldata after .bss, we
place .lrodata above .rodata to minimize the number of permission transitions in
the memory image.
While GNU ld places .lbss after .bss, the subsequent sections don't reuse the
file offset bytes of BSS.
Our approach is to place .ldata and .lbss after .bss and create a PT_LOAD
segment for .bss to large data section transition in the absence of SECTIONS
commands. assignFileOffsets ensures we insert an alignment instead of allocating
space for BSS, and therefore we don't waste more than MAXPAGESIZE bytes. We have
a missing optimization to prevent all waste, but implementing it would introduce
complexity and likely be error-prone.
GNU ld's layout introduces 2 more MAXPAGESIZE alignments while ours
introduces just one.
[1]: https://groups.google.com/g/x86-64-abi/c/jnQdJeabxiU "Large data sections for the large code model"
With help from Arthur Eubanks.
Co-authored-by: James Y Knight <jyknight at google.com>
Reviewed By: aeubanks, tkoeppe
Differential Revision: https://reviews.llvm.org/D150510
Added:
lld/test/ELF/x86-64-section-layout.s
Modified:
lld/ELF/LinkerScript.cpp
lld/ELF/Writer.cpp
Removed:
################################################################################
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 2f183ebc194c..885937196317 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -101,9 +101,9 @@ static StringRef getOutputSectionName(const InputSectionBase *s) {
}
for (StringRef v :
- {".data.rel.ro", ".data", ".rodata", ".bss.rel.ro", ".bss",
- ".gcc_except_table", ".init_array", ".fini_array", ".tbss", ".tdata",
- ".ARM.exidx", ".ARM.extab", ".ctors", ".dtors"})
+ {".data.rel.ro", ".data", ".rodata", ".bss.rel.ro", ".bss", ".ldata",
+ ".lrodata", ".lbss", ".gcc_except_table", ".init_array", ".fini_array",
+ ".tbss", ".tdata", ".ARM.exidx", ".ARM.extab", ".ctors", ".dtors"})
if (isSectionPrefix(v, s->name))
return v;
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 9c2205222165..f68d4f4c34ae 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -834,10 +834,11 @@ enum RankFlags {
RF_NOT_ALLOC = 1 << 26,
RF_PARTITION = 1 << 18, // Partition number (8 bits)
RF_NOT_SPECIAL = 1 << 17,
- RF_WRITE = 1 << 13,
- RF_EXEC_WRITE = 1 << 12,
- RF_EXEC = 1 << 11,
- RF_RODATA = 1 << 10,
+ RF_WRITE = 1 << 16,
+ RF_EXEC_WRITE = 1 << 15,
+ RF_EXEC = 1 << 14,
+ RF_RODATA = 1 << 13,
+ RF_LARGE = 1 << 12,
RF_NOT_RELRO = 1 << 9,
RF_NOT_TLS = 1 << 8,
RF_BSS = 1 << 7,
@@ -895,6 +896,9 @@ static unsigned getSectionRank(const OutputSection &osec) {
// .dynstr and .dynsym can be away from .text.
if (osec.type == SHT_PROGBITS)
rank |= RF_RODATA;
+ // Among PROGBITS sections, place .lrodata further from .text.
+ if (!(osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64))
+ rank |= RF_LARGE;
} else if (isExec) {
rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC;
} else {
@@ -905,6 +909,10 @@ static unsigned getSectionRank(const OutputSection &osec) {
rank |= RF_NOT_TLS;
if (!isRelroSection(&osec))
rank |= RF_NOT_RELRO;
+ // Place .ldata and .lbss after .bss. Making .bss closer to .text alleviates
+ // relocation overflow pressure.
+ if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64)
+ rank |= RF_LARGE;
}
// Within TLS sections, or within other RelRo sections, or within non-RelRo
@@ -2321,16 +2329,27 @@ SmallVector<PhdrEntry *, 0> Writer<ELFT>::createPhdrs(Partition &part) {
// Segments are contiguous memory regions that has the same attributes
// (e.g. executable or writable). There is one phdr for each segment.
// Therefore, we need to create a new phdr when the next section has
- //
diff erent flags or is loaded at a discontiguous address or memory
- // region using AT or AT> linker script command, respectively. At the same
- // time, we don't want to create a separate load segment for the headers,
- // even if the first output section has an AT or AT> attribute.
+ //
diff erent flags or is loaded at a discontiguous address or memory region
+ // using AT or AT> linker script command, respectively.
+ //
+ // As an exception, we don't create a separate load segment for the ELF
+ // headers, even if the first "real" output has an AT or AT> attribute.
+ //
+ // In addition, NOBITS sections should only be placed at the end of a LOAD
+ // segment (since it's represented as p_filesz < p_memsz). If we have a
+ // not-NOBITS section after a NOBITS, we create a new LOAD for the latter
+ // even if flags match, so as not to require actually writing the
+ // supposed-to-be-NOBITS section to the output file. (However, we cannot do
+ // so when hasSectionsCommand, since we cannot introduce the extra alignment
+ // needed to create a new LOAD)
uint64_t newFlags = computeFlags(sec->getPhdrFlags());
bool sameLMARegion =
load && !sec->lmaExpr && sec->lmaRegion == load->firstSec->lmaRegion;
if (!(load && newFlags == flags && sec != relroEnd &&
sec->memRegion == load->firstSec->memRegion &&
- (sameLMARegion || load->lastSec == Out::programHeaders))) {
+ (sameLMARegion || load->lastSec == Out::programHeaders) &&
+ (script->hasSectionsCommand || sec->type == SHT_NOBITS ||
+ load->lastSec->type != SHT_NOBITS))) {
load = addHdr(PT_LOAD, newFlags);
flags = newFlags;
}
diff --git a/lld/test/ELF/x86-64-section-layout.s b/lld/test/ELF/x86-64-section-layout.s
new file mode 100644
index 000000000000..e9ea69afab3b
--- /dev/null
+++ b/lld/test/ELF/x86-64-section-layout.s
@@ -0,0 +1,110 @@
+# REQUIRES: x86
+## Test the placement of .lrodata, .lbss, .ldata, and their -fdata-sections variants.
+## See also section-layout.s.
+
+# RUN: rm -rf %t && split-file %s %t && cd %t
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 --defsym=BSS=1 a.s -o a.o
+# RUN: ld.lld --section-start=.note=0x200300 a.o -o a
+# RUN: llvm-readelf -S -l a | FileCheck %s
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 a.s -o a1.o
+# RUN: ld.lld --section-start=.note=0x200300 a1.o -o a1
+# RUN: llvm-readelf -S a1 | FileCheck %s --check-prefix=CHECK1
+
+# RUN: ld.lld -T b.lds -z norelro a.o -o b
+# RUN: llvm-readelf -S -l b | FileCheck %s --check-prefix=CHECK2
+
+# CHECK: Name Type Address Off Size ES Flg Lk Inf Al
+# CHECK-NEXT: NULL 0000000000000000 000000 000000 00 0 0 0
+# CHECK-NEXT: .note NOTE 0000000000200300 000300 000001 00 A 0 0 1
+# CHECK-NEXT: .lrodata PROGBITS 0000000000200301 000301 000002 00 Al 0 0 1
+# CHECK-NEXT: .rodata PROGBITS 0000000000200303 000303 000001 00 A 0 0 1
+# CHECK-NEXT: .text PROGBITS 0000000000201304 000304 000001 00 AX 0 0 4
+# CHECK-NEXT: .tdata PROGBITS 0000000000202305 000305 000001 00 WAT 0 0 1
+# CHECK-NEXT: .tbss NOBITS 0000000000202306 000306 000002 00 WAT 0 0 1
+# CHECK-NEXT: .data PROGBITS 0000000000203306 000306 000001 00 WA 0 0 1
+# CHECK-NEXT: .bss NOBITS 0000000000203307 000307 001800 00 WA 0 0 1
+## We spend size(.bss) % MAXPAGESIZE bytes for .bss.
+# CHECK-NEXT: .ldata PROGBITS 0000000000205b07 000b07 000002 00 WAl 0 0 1
+# CHECK-NEXT: .ldata2 PROGBITS 0000000000205b09 000b09 000001 00 WAl 0 0 1
+# CHECK-NEXT: .lbss NOBITS 0000000000205b0a 000b0a 000002 00 WAl 0 0 1
+# CHECK-NEXT: .comment PROGBITS 0000000000000000 000b0a {{.*}} 01 MS 0 0 1
+
+# CHECK: Program Headers:
+# CHECK-NEXT: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
+# CHECK-NEXT: PHDR 0x000040 0x0000000000200040 0x0000000000200040 {{.*}} {{.*}} R 0x8
+# CHECK-NEXT: LOAD 0x000000 0x0000000000200000 0x0000000000200000 0x000304 0x000304 R 0x1000
+# CHECK-NEXT: LOAD 0x000304 0x0000000000201304 0x0000000000201304 0x000001 0x000001 R E 0x1000
+# CHECK-NEXT: LOAD 0x000305 0x0000000000202305 0x0000000000202305 0x000001 0x000001 RW 0x1000
+# CHECK-NEXT: LOAD 0x000306 0x0000000000203306 0x0000000000203306 0x000001 0x001801 RW 0x1000
+# CHECK-NEXT: LOAD 0x000b07 0x0000000000205b07 0x0000000000205b07 0x000003 0x000005 RW 0x1000
+
+# CHECK1: .data PROGBITS 0000000000203306 000306 000001 00 WA 0 0 1
+# CHECK1-NEXT: .ldata PROGBITS 0000000000203307 000307 000002 00 WAl 0 0 1
+# CHECK1-NEXT: .ldata2 PROGBITS 0000000000203309 000309 000001 00 WAl 0 0 1
+# CHECK1-NEXT: .comment PROGBITS 0000000000000000 00030a {{.*}} 01 MS 0 0 1
+
+# CHECK2: .note NOTE 0000000000200300 000300 000001 00 A 0 0 1
+# CHECK2-NEXT: .lrodata PROGBITS 0000000000200301 000301 000001 00 Al 0 0 1
+## With a SECTIONS command, we suppress the default rule placing .lrodata.* into .lrodata.
+# CHECK2-NEXT: .lrodata.1 PROGBITS 0000000000200302 000302 000001 00 Al 0 0 1
+# CHECK2-NEXT: .rodata PROGBITS 0000000000200303 000303 000001 00 A 0 0 1
+# CHECK2-NEXT: .text PROGBITS 0000000000200304 000304 000001 00 AX 0 0 4
+# CHECK2-NEXT: .tdata PROGBITS 0000000000200305 000305 000001 00 WAT 0 0 1
+# CHECK2-NEXT: .tbss NOBITS 0000000000200306 000306 000001 00 WAT 0 0 1
+# CHECK2-NEXT: .tbss.1 NOBITS 0000000000200307 000306 000001 00 WAT 0 0 1
+# CHECK2-NEXT: .data PROGBITS 0000000000200306 000306 000001 00 WA 0 0 1
+# CHECK2-NEXT: .bss NOBITS 0000000000200307 000307 001800 00 WA 0 0 1
+# CHECK2-NEXT: .ldata PROGBITS 0000000000201b07 001b07 000002 00 WAl 0 0 1
+# CHECK2-NEXT: .ldata2 PROGBITS 0000000000201b09 001b09 000001 00 WAl 0 0 1
+# CHECK2-NEXT: .lbss NOBITS 0000000000201b0a 001b0a 000002 00 WAl 0 0 1
+# CHECK2-NEXT: .comment PROGBITS 0000000000000000 001b0a {{.*}} 01 MS 0 0 1
+
+# CHECK2: Program Headers:
+# CHECK2-NEXT: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
+# CHECK2-NEXT: PHDR 0x000040 0x0000000000200040 0x0000000000200040 {{.*}} {{.*}} R 0x8
+# CHECK2-NEXT: LOAD 0x000000 0x0000000000200000 0x0000000000200000 0x000304 0x000304 R 0x1000
+# CHECK2-NEXT: LOAD 0x000304 0x0000000000200304 0x0000000000200304 0x000001 0x000001 R E 0x1000
+# CHECK2-NEXT: LOAD 0x000305 0x0000000000200305 0x0000000000200305 0x001805 0x001807 RW 0x1000
+# CHECK2-NEXT: TLS 0x000305 0x0000000000200305 0x0000000000200305 0x000001 0x000003 R 0x1
+
+#--- a.s
+.globl _start
+_start:
+ ret
+
+.section .note,"a", at note; .space 1
+.section .rodata,"a", at progbits; .space 1
+.section .data,"aw", at progbits; .space 1
+.ifdef BSS
+## .bss is large than one MAXPAGESIZE to test file offsets.
+.section .bss,"aw", at nobits; .space 0x1800
+.endif
+.section .tdata,"awT", at progbits; .space 1
+.section .tbss,"awT", at nobits; .space 1
+.section .tbss.1,"awT", at nobits; .space 1
+
+.section .lrodata,"al"; .space 1
+.section .lrodata.1,"al"; .space 1
+.section .ldata,"awl"; .space 1
+## Input .ldata.rel.ro sections are placed in the output .ldata section.
+.section .ldata.rel.ro,"awl"; .space 1
+.ifdef BSS
+.section .lbss,"awl", at nobits; .space 1
+## Input .lbss.rel.ro sections are placed in the output .lbss section.
+.section .lbss.rel.ro,"awl", at nobits; .space 1
+.endif
+.section .ldata2,"awl"; .space 1
+
+#--- b.lds
+SECTIONS {
+ . = 0x200300;
+ .rodata : {}
+ .text : {}
+ .data : {}
+ .bss : {}
+ .ldata : { *(.ldata .ldata.*) }
+ .ldata2 : {}
+ .lbss : { *(.lbss .lbss.*) }
+}
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