From 3b8d2be527259b303d6c3428df16fb3fd02af2bc Mon Sep 17 00:00:00 2001 From: Sam Clegg Date: Fri, 26 Feb 2021 16:09:32 -0800 Subject: Reland: "[lld][WebAssembly] Initial support merging string data" This change was originally landed in: 5000a1b4b9edeb9e994f2a5b36da8d48599bea49 It was reverted in: 061e071d8c9b98526f35cad55a918a4f1615afd4 This change adds support for a new WASM_SEG_FLAG_STRINGS flag in the object format which works in a similar fashion to SHF_STRINGS in the ELF world. Unlike the ELF linker this support is currently limited: - No support for SHF_MERGE (non-string merging) - Always do full tail merging ("lo" can be merged with "hello") - Only support single byte strings (p2align 0) Like the ELF linker merging is only performed at `-O1` and above. This fixes part of https://bugs.llvm.org/show_bug.cgi?id=48828, although crucially it doesn't not currently support debug sections because they are not represented by data segments (they are custom sections) Differential Revision: https://reviews.llvm.org/D97657 --- llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-) (limited to 'llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp') diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp index 3227fd7..54d65cc 100644 --- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp +++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp @@ -21,6 +21,7 @@ #include "llvm/BinaryFormat/Dwarf.h" #include "llvm/BinaryFormat/ELF.h" #include "llvm/BinaryFormat/MachO.h" +#include "llvm/BinaryFormat/Wasm.h" #include "llvm/CodeGen/BasicBlockSectionUtils.h" #include "llvm/CodeGen/MachineBasicBlock.h" #include "llvm/CodeGen/MachineFunction.h" @@ -2005,6 +2006,17 @@ static const Comdat *getWasmComdat(const GlobalValue *GV) { return C; } +static unsigned getWasmSectionFlags(SectionKind K) { + unsigned Flags = 0; + + // TODO(sbc): Add suport for K.isMergeableConst() + + if (K.isMergeableCString()) + Flags |= wasm::WASM_SEG_FLAG_STRINGS; + + return Flags; +} + MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal( const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const { // We don't support explict section names for functions in the wasm object @@ -2028,9 +2040,9 @@ MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal( Group = C->getName(); } - MCSectionWasm* Section = - getContext().getWasmSection(Name, Kind, Group, - MCContext::GenericSectionID); + unsigned Flags = getWasmSectionFlags(Kind); + MCSectionWasm *Section = getContext().getWasmSection( + Name, Kind, Flags, Group, MCContext::GenericSectionID); return Section; } @@ -2062,7 +2074,8 @@ static MCSectionWasm *selectWasmSectionForGlobal( (*NextUniqueID)++; } - return Ctx.getWasmSection(Name, Kind, Group, UniqueID); + unsigned Flags = getWasmSectionFlags(Kind); + return Ctx.getWasmSection(Name, Kind, Flags, Group, UniqueID); } MCSection *TargetLoweringObjectFileWasm::SelectSectionForGlobal( -- cgit v1.1