[llvm] [AArch64, ELF] Allow implicit $d/$x at section beginning (PR #99703)
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Fri Jul 19 13:39:09 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-mc
Author: Fangrui Song (MaskRay)
<details>
<summary>Changes</summary>
The start state of a new section is `EMS_None`, often leading to a
$d/$x at offset 0. Introduce a MCTargetOption/cl::opt
"optimize-mapping-symbols" to allow an alternative behavior:
* Set the start state to `EMS_Data` or `EMS_A64`.
* For text sections, add an ending $x only if the final data is not instructions.
* For non-text sections, add an ending $d only if the final data is not data commands.
```
.section .text1,"ax"
nop
// emit $d
.long 42
// emit $x
.section .text2,"ax"
nop
```
This new behavior decreases the .symtab size significantly:
```
% ~/projects/bloaty/out/debug/bloaty a64-2/bin/clang -- a64-0/bin/clang
FILE SIZE VM SIZE
-------------- --------------
-5.4% -1.13Mi [ = ] 0 .strtab
-50.9% -4.09Mi [ = ] 0 .symtab
-4.0% -5.22Mi [ = ] 0 TOTAL
```
---
This scheme works as long as the user can rule out some error scenarios:
* .text1 assembled using the traditional behavior is combined with .text2 using the new behavior
* A linker script combining non-text sections and text sections
---
Full diff: https://github.com/llvm/llvm-project/pull/99703.diff
5 Files Affected:
- (modified) llvm/include/llvm/MC/MCTargetOptions.h (+2)
- (modified) llvm/include/llvm/MC/MCTargetOptionsCommandFlags.h (+2)
- (modified) llvm/lib/MC/MCTargetOptionsCommandFlags.cpp (+7)
- (modified) llvm/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp (+33-5)
- (modified) llvm/test/MC/AArch64/mapping-across-sections.s (+10)
``````````diff
diff --git a/llvm/include/llvm/MC/MCTargetOptions.h b/llvm/include/llvm/MC/MCTargetOptions.h
index 899299fd15246..d16cb38fa0ae2 100644
--- a/llvm/include/llvm/MC/MCTargetOptions.h
+++ b/llvm/include/llvm/MC/MCTargetOptions.h
@@ -64,6 +64,8 @@ class MCTargetOptions {
// Use CREL relocation format for ELF.
bool Crel = false;
+ bool OptimizeMappingSymbols = false;
+
// If true, prefer R_X86_64_[REX_]GOTPCRELX to R_X86_64_GOTPCREL on x86-64
// ELF.
bool X86RelaxRelocations = true;
diff --git a/llvm/include/llvm/MC/MCTargetOptionsCommandFlags.h b/llvm/include/llvm/MC/MCTargetOptionsCommandFlags.h
index 9d592446f3ba7..f4b4e2cbcb450 100644
--- a/llvm/include/llvm/MC/MCTargetOptionsCommandFlags.h
+++ b/llvm/include/llvm/MC/MCTargetOptionsCommandFlags.h
@@ -53,6 +53,8 @@ bool getSaveTempLabels();
bool getCrel();
+bool getOptimizeMappingSymbols();
+
bool getX86RelaxRelocations();
bool getX86Sse2Avx();
diff --git a/llvm/lib/MC/MCTargetOptionsCommandFlags.cpp b/llvm/lib/MC/MCTargetOptionsCommandFlags.cpp
index 813b1194b47cb..f3dcf499eb9f9 100644
--- a/llvm/lib/MC/MCTargetOptionsCommandFlags.cpp
+++ b/llvm/lib/MC/MCTargetOptionsCommandFlags.cpp
@@ -48,6 +48,7 @@ MCOPT(bool, NoDeprecatedWarn)
MCOPT(bool, NoTypeCheck)
MCOPT(bool, SaveTempLabels)
MCOPT(bool, Crel)
+MCOPT(bool, OptimizeMappingSymbols)
MCOPT(bool, X86RelaxRelocations)
MCOPT(bool, X86Sse2Avx)
MCOPT(std::string, ABIName)
@@ -134,6 +135,11 @@ llvm::mc::RegisterMCTargetOptionsFlags::RegisterMCTargetOptionsFlags() {
cl::desc("Use CREL relocation format for ELF"));
MCBINDOPT(Crel);
+ static cl::opt<bool> OptimizeMappingSymbols(
+ "optimize-mapping-symbols",
+ cl::desc("Allow mapping symbol at section beginning to be implicit"));
+ MCBINDOPT(OptimizeMappingSymbols);
+
static cl::opt<bool> X86RelaxRelocations(
"x86-relax-relocations",
cl::desc(
@@ -174,6 +180,7 @@ MCTargetOptions llvm::mc::InitMCTargetOptionsFromFlags() {
Options.MCNoTypeCheck = getNoTypeCheck();
Options.MCSaveTempLabels = getSaveTempLabels();
Options.Crel = getCrel();
+ Options.OptimizeMappingSymbols = getOptimizeMappingSymbols();
Options.X86RelaxRelocations = getX86RelaxRelocations();
Options.X86Sse2Avx = getX86Sse2Avx();
Options.EmitDwarfUnwind = getEmitDwarfUnwind();
diff --git a/llvm/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp b/llvm/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp
index bfeca4bd5a92d..6bd118b39e043 100644
--- a/llvm/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp
+++ b/llvm/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp
@@ -32,8 +32,10 @@
#include "llvm/MC/MCStreamer.h"
#include "llvm/MC/MCSubtargetInfo.h"
#include "llvm/MC/MCSymbolELF.h"
+#include "llvm/MC/MCTargetOptions.h"
#include "llvm/MC/MCWinCOFFStreamer.h"
#include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
#include "llvm/Support/FormattedStream.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/TargetParser/Triple.h"
@@ -176,19 +178,29 @@ void AArch64TargetAsmStreamer::emitInst(uint32_t Inst) {
/// by MachO. Beware!
class AArch64ELFStreamer : public MCELFStreamer {
public:
+ friend AArch64TargetELFStreamer;
AArch64ELFStreamer(MCContext &Context, std::unique_ptr<MCAsmBackend> TAB,
std::unique_ptr<MCObjectWriter> OW,
std::unique_ptr<MCCodeEmitter> Emitter)
: MCELFStreamer(Context, std::move(TAB), std::move(OW),
std::move(Emitter)),
- MappingSymbolCounter(0), LastEMS(EMS_None) {}
+ MappingSymbolCounter(0), LastEMS(EMS_None) {
+ auto *TO = getContext().getTargetOptions();
+ OptimizeMappingSymbols = TO && TO->OptimizeMappingSymbols;
+ }
void changeSection(MCSection *Section, uint32_t Subsection = 0) override {
- // We have to keep track of the mapping symbol state of any sections we
- // use. Each one should start off as EMS_None, which is provided as the
- // default constructor by DenseMap::lookup.
+ // We have to keep track of the mapping symbol state of any sections we use.
+ // The initial state is EMS_A64 for text sections and EMS_Data for the
+ // others.
LastMappingSymbols[getCurrentSection().first] = LastEMS;
- LastEMS = LastMappingSymbols.lookup(Section);
+ auto It = LastMappingSymbols.find(Section);
+ if (It != LastMappingSymbols.end())
+ LastEMS = It->second;
+ else if (OptimizeMappingSymbols)
+ LastEMS = Section->isText() ? EMS_A64 : EMS_Data;
+ else
+ LastEMS = EMS_None;
MCELFStreamer::changeSection(Section, Subsection);
}
@@ -280,6 +292,7 @@ class AArch64ELFStreamer : public MCELFStreamer {
}
int64_t MappingSymbolCounter;
+ bool OptimizeMappingSymbols;
DenseMap<const MCSection *, ElfMappingSymbol> LastMappingSymbols;
ElfMappingSymbol LastEMS;
@@ -304,6 +317,21 @@ void AArch64TargetELFStreamer::finish() {
AArch64ELFStreamer &S = getStreamer();
MCContext &Ctx = S.getContext();
auto &Asm = S.getAssembler();
+
+ // If OptimizeMappingSymbols is specified, ensure that text sections end with
+ // the A64 state while non-text sections end with the data state. When
+ // sections are combined by the linker, the subsequent section will start with
+ // the right tate.
+ if (S.OptimizeMappingSymbols) {
+ for (MCSection &Sec : Asm) {
+ S.switchSection(&Sec);
+ if (Sec.isText())
+ S.emitA64MappingSymbol();
+ else
+ S.emitDataMappingSymbol();
+ }
+ }
+
MCSectionELF *MemtagSec = nullptr;
for (const MCSymbol &Symbol : Asm.symbols()) {
const auto &Sym = cast<MCSymbolELF>(Symbol);
diff --git a/llvm/test/MC/AArch64/mapping-across-sections.s b/llvm/test/MC/AArch64/mapping-across-sections.s
index 6bb5a8811b57d..49151326caf6c 100644
--- a/llvm/test/MC/AArch64/mapping-across-sections.s
+++ b/llvm/test/MC/AArch64/mapping-across-sections.s
@@ -1,4 +1,5 @@
// RUN: llvm-mc -triple=aarch64 -filetype=obj %s | llvm-objdump -t - | FileCheck %s
+// RUN: llvm-mc -triple=aarch64 -filetype=obj -optimize-mapping-symbols %s | llvm-objdump -t - | FileCheck %s --check-prefix=CHECK1
.section .text1,"ax"
add w0, w0, w0
@@ -38,3 +39,12 @@ add w0, w0, w0
// CHECK-NEXT: 0000000000000004 l .rodata 0000000000000000 $x.7
// CHECK-NEXT: 0000000000000000 l .comment 0000000000000000 $d.8
// CHECK-NOT: {{.}}
+
+// CHECK1: SYMBOL TABLE:
+// CHECK1-NEXT: 0000000000000004 l .text 0000000000000000 $d.0
+// CHECK1-NEXT: 0000000000000008 l .text 0000000000000000 $x.1
+// CHECK1-NEXT: 000000000000000c l .text 0000000000000000 $d.2
+// CHECK1-NEXT: 0000000000000004 l .rodata 0000000000000000 $x.3
+// CHECK1-NEXT: 0000000000000010 l .text 0000000000000000 $x.4
+// CHECK1-NEXT: 0000000000000008 l .rodata 0000000000000000 $d.5
+// CHECK-NOT: {{.}}
``````````
</details>
https://github.com/llvm/llvm-project/pull/99703
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