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ShivaChenShivaChen
committedApr 11, 2019
[RISCV] Put data smaller than eight bytes to small data section
Because of gp = sdata_start_address + 0x800, gp with signed twelve-bit offset could covert most of the small data section. Linker relaxation could transfer the multiple data accessing instructions to a gp base with signed twelve-bit offset instruction. Differential Revision: https://reviews.llvm.org/D57493 llvm-svn: 358150
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‎llvm/include/llvm/Target/TargetLoweringObjectFile.h

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@@ -79,6 +79,9 @@ class TargetLoweringObjectFile : public MCObjectFileInfo {
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/// Emit the module-level metadata that the platform cares about.
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virtual void emitModuleMetadata(MCStreamer &Streamer, Module &M) const {}
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/// Get the module-level metadata that the platform cares about.
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virtual void getModuleMetadata(Module &M) {}
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/// Given a constant with the SectionKind, return a section that it should be
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/// placed in.
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virtual MCSection *getSectionForConstant(const DataLayout &DL,

‎llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

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@@ -259,6 +259,9 @@ bool AsmPrinter::doInitialization(Module &M) {
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const_cast<TargetLoweringObjectFile&>(getObjFileLowering())
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.Initialize(OutContext, TM);
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const_cast<TargetLoweringObjectFile &>(getObjFileLowering())
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.getModuleMetadata(M);
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OutStreamer->InitSections(false);
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// Emit the version-min deployment target directive if needed.

‎llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp

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@@ -8,11 +8,107 @@
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#include "RISCVTargetObjectFile.h"
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#include "RISCVTargetMachine.h"
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#include "llvm/BinaryFormat/ELF.h"
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#include "llvm/MC/MCContext.h"
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#include "llvm/MC/MCSectionELF.h"
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using namespace llvm;
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void RISCVELFTargetObjectFile::Initialize(MCContext &Ctx,
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const TargetMachine &TM) {
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TargetLoweringObjectFileELF::Initialize(Ctx, TM);
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InitializeELF(TM.Options.UseInitArray);
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SmallDataSection = getContext().getELFSection(
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".sdata", ELF::SHT_PROGBITS, ELF::SHF_WRITE | ELF::SHF_ALLOC);
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SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS,
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ELF::SHF_WRITE | ELF::SHF_ALLOC);
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}
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// A address must be loaded from a small section if its size is less than the
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// small section size threshold. Data in this section could be addressed by
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// using gp_rel operator.
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bool RISCVELFTargetObjectFile::isInSmallSection(uint64_t Size) const {
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// gcc has traditionally not treated zero-sized objects as small data, so this
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// is effectively part of the ABI.
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return Size > 0 && Size <= SSThreshold;
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}
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// Return true if this global address should be placed into small data/bss
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// section.
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bool RISCVELFTargetObjectFile::isGlobalInSmallSection(
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const GlobalObject *GO, const TargetMachine &TM) const {
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// Only global variables, not functions.
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const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GO);
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if (!GVA)
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return false;
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// If the variable has an explicit section, it is placed in that section.
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if (GVA->hasSection()) {
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StringRef Section = GVA->getSection();
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// Explicitly placing any variable in the small data section overrides
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// the global -G value.
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if (Section == ".sdata" || Section == ".sbss")
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return true;
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// Otherwise reject putting the variable to small section if it has an
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// explicit section name.
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return false;
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}
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if (((GVA->hasExternalLinkage() && GVA->isDeclaration()) ||
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GVA->hasCommonLinkage()))
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return false;
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Type *Ty = GVA->getValueType();
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// It is possible that the type of the global is unsized, i.e. a declaration
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// of a extern struct. In this case don't presume it is in the small data
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// section. This happens e.g. when building the FreeBSD kernel.
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if (!Ty->isSized())
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return false;
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return isInSmallSection(
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GVA->getParent()->getDataLayout().getTypeAllocSize(Ty));
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}
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MCSection *RISCVELFTargetObjectFile::SelectSectionForGlobal(
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const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
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// Handle Small Section classification here.
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if (Kind.isBSS() && isGlobalInSmallSection(GO, TM))
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return SmallBSSSection;
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if (Kind.isData() && isGlobalInSmallSection(GO, TM))
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return SmallDataSection;
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// Otherwise, we work the same as ELF.
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return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
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}
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void RISCVELFTargetObjectFile::getModuleMetadata(Module &M) {
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SmallVector<Module::ModuleFlagEntry, 8> ModuleFlags;
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M.getModuleFlagsMetadata(ModuleFlags);
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for (const auto &MFE : ModuleFlags) {
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StringRef Key = MFE.Key->getString();
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if (Key == "SmallDataLimit") {
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SSThreshold = mdconst::extract<ConstantInt>(MFE.Val)->getZExtValue();
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break;
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}
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}
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}
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/// Return true if this constant should be placed into small data section.
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bool RISCVELFTargetObjectFile::isConstantInSmallSection(
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const DataLayout &DL, const Constant *CN) const {
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return isInSmallSection(DL.getTypeAllocSize(CN->getType()));
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}
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MCSection *RISCVELFTargetObjectFile::getSectionForConstant(
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const DataLayout &DL, SectionKind Kind, const Constant *C,
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unsigned &Align) const {
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if (isConstantInSmallSection(DL, C))
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return SmallDataSection;
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// Otherwise, we work the same as ELF.
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return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C, Align);
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}

‎llvm/lib/Target/RISCV/RISCVTargetObjectFile.h

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@@ -16,7 +16,31 @@ class RISCVTargetMachine;
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/// This implementation is used for RISCV ELF targets.
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class RISCVELFTargetObjectFile : public TargetLoweringObjectFileELF {
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MCSection *SmallDataSection;
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MCSection *SmallBSSSection;
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unsigned SSThreshold = 8;
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public:
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void Initialize(MCContext &Ctx, const TargetMachine &TM) override;
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/// Return true if this global address should be placed into small data/bss
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/// section.
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bool isGlobalInSmallSection(const GlobalObject *GO,
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const TargetMachine &TM) const;
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MCSection *SelectSectionForGlobal(const GlobalObject *GO, SectionKind Kind,
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const TargetMachine &TM) const override;
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/// Return true if this constant should be placed into small data section.
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bool isConstantInSmallSection(const DataLayout &DL, const Constant *CN) const;
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MCSection *getSectionForConstant(const DataLayout &DL, SectionKind Kind,
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const Constant *C,
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unsigned &Align) const override;
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void getModuleMetadata(Module &M) override;
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bool isInSmallSection(uint64_t Size) const;
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};
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} // end namespace llvm

‎llvm/test/CodeGen/RISCV/sdata-limit-0.ll

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; RUN: llc -mtriple=riscv32 < %s | FileCheck -check-prefix=RV32 %s
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; RUN: llc -mtriple=riscv64 < %s | FileCheck -check-prefix=RV64 %s
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@v = dso_local global i32 0, align 4
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@r = dso_local global i64 7, align 8
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; SmallDataLimit set to 0, so we expect no data will put in sbss and sdata.
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!llvm.module.flags = !{!0}
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!0 = !{i32 1, !"SmallDataLimit", i32 0}
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; RV32-NOT: .section .sbss
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; RV32-NOT: .section .sdata
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; RV64-NOT: .section .sbss
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; RV64-NOT: .section .sdata

‎llvm/test/CodeGen/RISCV/sdata-limit-4.ll

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; RUN: llc -mtriple=riscv32 < %s | FileCheck -check-prefix=RV32 %s
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; RUN: llc -mtriple=riscv64 < %s | FileCheck -check-prefix=RV64 %s
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@v = dso_local global i32 0, align 4
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@r = dso_local global i64 7, align 8
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; SmallDataLimit set to 4, so we expect @v will be put in sbss,
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; but @r won't be put in sdata.
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!llvm.module.flags = !{!0}
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!0 = !{i32 1, !"SmallDataLimit", i32 4}
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; RV32: .section .sbss
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; RV32-NOT: .section .sdata
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; RV64: .section .sbss
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; RV64-NOT: .section .sdata

‎llvm/test/CodeGen/RISCV/sdata-limit-8.ll

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; RUN: llc -mtriple=riscv32 < %s | FileCheck -check-prefix=RV32 %s
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; RUN: llc -mtriple=riscv64 < %s | FileCheck -check-prefix=RV64 %s
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@v = dso_local global i32 0, align 4
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@r = dso_local global i64 7, align 8
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; SmallDataLimit set to 8, so we expect @v will be put in sbss
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; and @r will be put in sdata.
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!llvm.module.flags = !{!0}
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!0 = !{i32 1, !"SmallDataLimit", i32 8}
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; RV32: .section .sbss
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; RV32: .section .sdata
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; RV64: .section .sbss
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; RV64: .section .sdata

‎llvm/test/CodeGen/RISCV/sdata-local-sym.ll

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; RUN: llc -mtriple=riscv32 < %s | FileCheck -check-prefix=RV32 %s
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; RUN: llc -mtriple=riscv64 < %s | FileCheck -check-prefix=RV64 %s
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@v = internal global i32 0, align 4
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@r = internal global i64 7, align 8
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; @v and @r are local symbols.
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; SmallDataLimit set to 8, so we expect @v will be put in sbss
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; and @r will be put in sdata.
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!llvm.module.flags = !{!0}
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!0 = !{i32 1, !"SmallDataLimit", i32 8}
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; RV32: .section .sbss
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; RV32: .section .sdata
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; RV64: .section .sbss
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; RV64: .section .sdata

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