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[lld/ELF][x86-64] Place large executable sections at the edges of binary #70358

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Jun 12, 2025
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[lld/ELF][x86-64] Place large executable sections at the edges of binary #70358

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0389a49
[lld/ELF] Place large executable sections at beginning or end
aeubanks May 29, 2025
8667a29
address comments
aeubanks Jun 12, 2025
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23 changes: 16 additions & 7 deletions lld/ELF/Writer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -653,15 +653,17 @@ enum RankFlags {
RF_NOT_ADDR_SET = 1 << 27,
RF_NOT_ALLOC = 1 << 26,
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This ranking mechanism is a bit difficult to understand, but changing it is out of scope of this change.

RF_PARTITION = 1 << 18, // Partition number (8 bits)
RF_LARGE_EXEC_WRITE = 1 << 16,
RF_LARGE_ALT = 1 << 15,
RF_WRITE = 1 << 14,
RF_EXEC_WRITE = 1 << 13,
RF_EXEC = 1 << 12,
RF_RODATA = 1 << 11,
RF_LARGE = 1 << 10,
RF_NOT_RELRO = 1 << 9,
RF_NOT_TLS = 1 << 8,
RF_BSS = 1 << 7,
RF_LARGE_EXEC = 1 << 10,
RF_LARGE = 1 << 9,
RF_NOT_RELRO = 1 << 8,
RF_NOT_TLS = 1 << 7,
RF_BSS = 1 << 6,
};

unsigned elf::getSectionRank(Ctx &ctx, OutputSection &osec) {
Expand Down Expand Up @@ -691,14 +693,15 @@ unsigned elf::getSectionRank(Ctx &ctx, OutputSection &osec) {
// places.
bool isExec = osec.flags & SHF_EXECINSTR;
bool isWrite = osec.flags & SHF_WRITE;
bool isLarge = osec.flags & SHF_X86_64_LARGE && ctx.arg.emachine == EM_X86_64;

if (!isWrite && !isExec) {
// Among PROGBITS sections, place .lrodata further from .text.
// For -z lrodata-after-bss, place .lrodata after .lbss like GNU ld. This
// layout has one extra PT_LOAD, but alleviates relocation overflow
// pressure for absolute relocations referencing small data from -fno-pic
// relocatable files.
if (osec.flags & SHF_X86_64_LARGE && ctx.arg.emachine == EM_X86_64)
if (isLarge)
rank |= ctx.arg.zLrodataAfterBss ? RF_LARGE_ALT : 0;
else
rank |= ctx.arg.zLrodataAfterBss ? 0 : RF_LARGE;
Expand All @@ -722,7 +725,13 @@ unsigned elf::getSectionRank(Ctx &ctx, OutputSection &osec) {
else
rank |= RF_RODATA;
} else if (isExec) {
rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC;
// Place readonly .ltext before .lrodata and writable .ltext after .lbss to
// keep writable and readonly segments separate.
if (isLarge) {
rank |= isWrite ? RF_LARGE_EXEC_WRITE : RF_LARGE_EXEC;
} else {
rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC;
}
} else {
rank |= RF_WRITE;
// The TLS initialization block needs to be a single contiguous block. Place
Expand All @@ -737,7 +746,7 @@ unsigned elf::getSectionRank(Ctx &ctx, OutputSection &osec) {
// alleviates relocation overflow pressure.
// For -z lrodata-after-bss, place .lbss/.lrodata/.ldata after .bss.
// .bss/.lbss being adjacent reuses the NOBITS size optimization.
if (osec.flags & SHF_X86_64_LARGE && ctx.arg.emachine == EM_X86_64) {
if (isLarge) {
rank |= ctx.arg.zLrodataAfterBss
? (osec.type == SHT_NOBITS ? 1 : RF_LARGE_ALT)
: RF_LARGE;
Expand Down
41 changes: 41 additions & 0 deletions lld/test/ELF/x86-64-section-layout.s
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,10 @@
# RUN: ld.lld --section-start=.note=0x200300 a.o -z lrodata-after-bss -o a3
# RUN: llvm-readelf -S -l -sX a3 | FileCheck %s --check-prefix=CHECK3

# RUN: llvm-mc -filetype=obj -triple=x86_64 c.s -o c.o
# RUN: ld.lld c.o -o c
# RUN: llvm-readelf -S -l c | FileCheck %s --check-prefix=CHECK4

# 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
Expand Down Expand Up @@ -116,6 +120,18 @@
# CHECK3-NEXT: 0000000000203307 0 NOTYPE GLOBAL DEFAULT [[#]] (.data) _edata
# CHECK3-NEXT: 0000000000207d0d 0 NOTYPE GLOBAL DEFAULT [[#]] (.ldata2) _end

# CHECK4: .note NOTE
# CHECK4-NEXT: .ltext PROGBITS
# CHECK4-NEXT: .lrodata PROGBITS
# CHECK4-NEXT: .rodata PROGBITS
# CHECK4-NEXT: .text PROGBITS
# CHECK4-NEXT: .data PROGBITS
# CHECK4-NEXT: .bss NOBITS
# CHECK4-NEXT: .ldata PROGBITS
# CHECK4-NEXT: .lbss NOBITS
# CHECK4-NEXT: .ltext_w PROGBITS
# CHECK4-NEXT: .comment PROGBITS

#--- a.s
.globl _start, _etext, _edata, _end
_start:
Expand Down Expand Up @@ -155,3 +171,28 @@ SECTIONS {
.ldata2 : {}
.lbss : { *(.lbss .lbss.*) }
}

#--- c.s
## Test .ltext layout
.section .ltext,"axl",@progbits
.globl f
f:
ret

.section .ltext_w,"awxl",@progbits
.globl g
g:
ret

.section .text,"ax",@progbits
.globl h
h:
ret

.section .note,"a",@note; .space 1
.section .rodata,"a",@progbits; .space 1
.section .data,"aw",@progbits; .space 1
.section .bss,"aw",@nobits; .space 1
.section .lrodata,"al"; .space 1
.section .ldata,"awl"; .space 1
.section .lbss,"awl",@nobits; .space 1
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