[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
diff --git a/llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp b/llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp
index ec61ab7..bbd45c9 100644
--- a/llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp
+++ b/llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp
@@ -8,6 +8,9 @@
#include "RISCVTargetObjectFile.h"
#include "RISCVTargetMachine.h"
+#include "llvm/BinaryFormat/ELF.h"
+#include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCSectionELF.h"
using namespace llvm;
@@ -15,4 +18,97 @@
const TargetMachine &TM) {
TargetLoweringObjectFileELF::Initialize(Ctx, TM);
InitializeELF(TM.Options.UseInitArray);
+
+ SmallDataSection = getContext().getELFSection(
+ ".sdata", ELF::SHT_PROGBITS, ELF::SHF_WRITE | ELF::SHF_ALLOC);
+ SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS,
+ ELF::SHF_WRITE | ELF::SHF_ALLOC);
+}
+
+// A address must be loaded from a small section if its size is less than the
+// small section size threshold. Data in this section could be addressed by
+// using gp_rel operator.
+bool RISCVELFTargetObjectFile::isInSmallSection(uint64_t Size) const {
+ // gcc has traditionally not treated zero-sized objects as small data, so this
+ // is effectively part of the ABI.
+ return Size > 0 && Size <= SSThreshold;
+}
+
+// Return true if this global address should be placed into small data/bss
+// section.
+bool RISCVELFTargetObjectFile::isGlobalInSmallSection(
+ const GlobalObject *GO, const TargetMachine &TM) const {
+ // Only global variables, not functions.
+ const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GO);
+ if (!GVA)
+ return false;
+
+ // If the variable has an explicit section, it is placed in that section.
+ if (GVA->hasSection()) {
+ StringRef Section = GVA->getSection();
+
+ // Explicitly placing any variable in the small data section overrides
+ // the global -G value.
+ if (Section == ".sdata" || Section == ".sbss")
+ return true;
+
+ // Otherwise reject putting the variable to small section if it has an
+ // explicit section name.
+ return false;
+ }
+
+ if (((GVA->hasExternalLinkage() && GVA->isDeclaration()) ||
+ GVA->hasCommonLinkage()))
+ return false;
+
+ Type *Ty = GVA->getValueType();
+ // It is possible that the type of the global is unsized, i.e. a declaration
+ // of a extern struct. In this case don't presume it is in the small data
+ // section. This happens e.g. when building the FreeBSD kernel.
+ if (!Ty->isSized())
+ return false;
+
+ return isInSmallSection(
+ GVA->getParent()->getDataLayout().getTypeAllocSize(Ty));
+}
+
+MCSection *RISCVELFTargetObjectFile::SelectSectionForGlobal(
+ const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
+ // Handle Small Section classification here.
+ if (Kind.isBSS() && isGlobalInSmallSection(GO, TM))
+ return SmallBSSSection;
+ if (Kind.isData() && isGlobalInSmallSection(GO, TM))
+ return SmallDataSection;
+
+ // Otherwise, we work the same as ELF.
+ return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
+}
+
+void RISCVELFTargetObjectFile::getModuleMetadata(Module &M) {
+ SmallVector<Module::ModuleFlagEntry, 8> ModuleFlags;
+ M.getModuleFlagsMetadata(ModuleFlags);
+
+ for (const auto &MFE : ModuleFlags) {
+ StringRef Key = MFE.Key->getString();
+ if (Key == "SmallDataLimit") {
+ SSThreshold = mdconst::extract<ConstantInt>(MFE.Val)->getZExtValue();
+ break;
+ }
+ }
+}
+
+/// Return true if this constant should be placed into small data section.
+bool RISCVELFTargetObjectFile::isConstantInSmallSection(
+ const DataLayout &DL, const Constant *CN) const {
+ return isInSmallSection(DL.getTypeAllocSize(CN->getType()));
+}
+
+MCSection *RISCVELFTargetObjectFile::getSectionForConstant(
+ const DataLayout &DL, SectionKind Kind, const Constant *C,
+ unsigned &Align) const {
+ if (isConstantInSmallSection(DL, C))
+ return SmallDataSection;
+
+ // Otherwise, we work the same as ELF.
+ return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C, Align);
}