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[MC] Attach labels to existing fragments instead of using a separate fragment

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Summary:
Currently when emitting a label, a new data fragment is created for it if the
current fragment isn't a data fragment.
This change instead enqueues the label and attaches it to the next fragment
(e.g. created for the next instruction) if possible.

When bundle alignment is not enabled, this has no functionality change (it
just results in fewer extra fragments being created). For bundle alignment,
previously labels would point to the beginning of the bundle padding instead
of the beginning of the emitted instruction. This was not only less efficient
(e.g. jumping to the nops instead of past them) but also led to miscalculation
of the address of the GOT (since MC uses a label difference rather than
emitting a "." symbol).

Fixes https://code.google.com/p/nativeclient/issues/detail?id=3982

Test Plan: regression test attached

Reviewers: jvoung, eliben

Subscribers: jfb, llvm-commits

Differential Revision: http://reviews.llvm.org/D5915

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220439 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Derek Schuff 2014-10-22 22:38:06 +00:00
parent 545127f54d
commit cdb105b62f
4 changed files with 122 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
include/llvm/MC/MCObjectStreamer.h
View File

@ -10,6 +10,7 @@
#ifndef LLVM_MC_MCOBJECTSTREAMER_H
#define LLVM_MC_MCOBJECTSTREAMER_H
#include "llvm/ADT/SmallVector.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCStreamer.h"
@ -37,11 +38,16 @@ class MCObjectStreamer : public MCStreamer {
MCSectionData::iterator CurInsertionPoint;
bool EmitEHFrame;
bool EmitDebugFrame;
SmallVector<MCSymbolData *, 2> PendingLabels;
virtual void EmitInstToData(const MCInst &Inst, const MCSubtargetInfo&) = 0;
void EmitCFIStartProcImpl(MCDwarfFrameInfo &Frame) override;
void EmitCFIEndProcImpl(MCDwarfFrameInfo &Frame) override;
// If any labels have been emitted but not assigned fragments, ensure that
// they get assigned, either to F if possible or to a new data fragment.
void flushPendingLabels(MCFragment *F);
protected:
MCObjectStreamer(MCContext &Context, MCAsmBackend &TAB, raw_ostream &_OS,
MCCodeEmitter *_Emitter);
@ -69,14 +75,15 @@ protected:
MCFragment *getCurrentFragment() const;
void insert(MCFragment *F) const {
void insert(MCFragment *F) {
flushPendingLabels(F);
CurSectionData->getFragmentList().insert(CurInsertionPoint, F);
F->setParent(CurSectionData);
}
/// Get a data fragment to write into, creating a new one if the current
/// fragment is not a data fragment.
MCDataFragment *getOrCreateDataFragment() const;
MCDataFragment *getOrCreateDataFragment();
public:
void visitUsedSymbol(const MCSymbol &Sym) override;

38
lib/MC/MCObjectStreamer.cpp
View File

@ -42,6 +42,21 @@ MCObjectStreamer::~MCObjectStreamer() {
delete Assembler;
}
void MCObjectStreamer::flushPendingLabels(MCFragment *F) {
if (PendingLabels.size()) {
if (!F) {
F = new MCDataFragment();
CurSectionData->getFragmentList().insert(CurInsertionPoint, F);
F->setParent(CurSectionData);
}
for (MCSymbolData *SD : PendingLabels) {
SD->setFragment(F);
SD->setOffset(0);
}
PendingLabels.clear();
}
}
void MCObjectStreamer::reset() {
if (Assembler)
Assembler->reset();
@ -49,6 +64,7 @@ void MCObjectStreamer::reset() {
CurInsertionPoint = MCSectionData::iterator();
EmitEHFrame = true;
EmitDebugFrame = false;
PendingLabels.clear();
MCStreamer::reset();
}
@ -72,7 +88,7 @@ MCFragment *MCObjectStreamer::getCurrentFragment() const {
return nullptr;
}
MCDataFragment *MCObjectStreamer::getOrCreateDataFragment() const {
MCDataFragment *MCObjectStreamer::getOrCreateDataFragment() {
MCDataFragment *F = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
// When bundling is enabled, we don't want to add data to a fragment that
// already has instructions (see MCELFStreamer::EmitInstToData for details)
@ -127,15 +143,17 @@ void MCObjectStreamer::EmitLabel(MCSymbol *Symbol) {
MCStreamer::EmitLabel(Symbol);
MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
// FIXME: This is wasteful, we don't necessarily need to create a data
// fragment. Instead, we should mark the symbol as pointing into the data
// fragment if it exists, otherwise we should just queue the label and set its
// fragment pointer when we emit the next fragment.
MCDataFragment *F = getOrCreateDataFragment();
assert(!SD.getFragment() && "Unexpected fragment on symbol data!");
SD.setFragment(F);
SD.setOffset(F->getContents().size());
// If there is a current fragment, mark the symbol as pointing into it.
// Otherwise queue the label and set its fragment pointer when we emit the
// next fragment.
if (dyn_cast_or_null<MCDataFragment>(getCurrentFragment())) {
SD.setFragment(F);
SD.setOffset(F->getContents().size());
} else {
PendingLabels.push_back(&SD);
}
}
void MCObjectStreamer::EmitULEB128Value(const MCExpr *Value) {
@ -164,6 +182,7 @@ void MCObjectStreamer::EmitWeakReference(MCSymbol *Alias,
void MCObjectStreamer::ChangeSection(const MCSection *Section,
const MCExpr *Subsection) {
assert(Section && "Cannot switch to a null section!");
flushPendingLabels(nullptr);
CurSectionData = &getAssembler().getOrCreateSectionData(*Section);
@ -398,5 +417,6 @@ void MCObjectStreamer::FinishImpl() {
// Dump out the dwarf file & directory tables and line tables.
MCDwarfLineTable::Emit(this);
flushPendingLabels(nullptr);
getAssembler().Finish();
}

83
test/MC/X86/AlignedBundling/labeloffset.s Normal file
View File

@ -0,0 +1,83 @@
# RUN: llvm-mc -triple=i686-linux -filetype=obj %s -o - | \
# RUN: llvm-objdump -disassemble -no-show-raw-insn -r - | FileCheck %s
# RUN: llvm-mc -triple=i686-nacl -filetype=obj %s -o - | \
# RUN: llvm-objdump -disassemble -no-show-raw-insn -r - | FileCheck %s
.bundle_align_mode 5
.text
.globl main
.align 32, 0x90
.type main,@function
main: # @main
# CHECK-LABEL: main:
# Call + pop sequence for determining the PIC base.
.bundle_lock align_to_end
calll .L0$pb
.bundle_unlock
.L0$pb:
popl %eax
# CHECK: 20: popl
# 26 bytes of instructions between the pop and the use of the pic base symbol.
movl $3, 2(%ebx, %ebx)
movl $3, 2(%ebx, %ebx)
movl $3, 2(%ebx, %ebx)
hlt
hlt
# CHECK: nop
.Ltmp0:
addl (.Ltmp0-.L0$pb), %eax
# The addl has bundle padding to push it from 0x3b to 0x40.
# The difference between the labels should be 0x20 (0x40-0x20) not 0x1b
# (0x3b-0x20)
# CHECK: 40: addl 32, %eax
popl %ecx
jmp *%ecx
# Also make sure it works with a non-relaxable instruction (cmp vs add)
# and for 2 adjacent labels that both point to the correct instruction
.section .text.bar, "ax"
.globl bar
.align 32, 0x90
.type bar,@function
bar:
# CHECK-LABEL: bar:
.bundle_lock align_to_end
calll .L1$pb
.bundle_unlock
.L1$pb:
popl %eax
# CHECK: 20: popl
# 26 bytes of instructions between the pop and the use of the pic base symbol.
movl $3, 2(%ebx, %ebx)
movl $3, 2(%ebx, %ebx)
movl $3, 2(%ebx, %ebx)
hlt
hlt
# CHECK: nop
.Ltmp1:
.Ltmp2:
cmpl %eax, .Ltmp1
# CHECK: 40: cmpl %eax, 64
cmpl %eax, (.Ltmp2-.L1$pb)
# CHECK: 46: cmpl %eax, 32
popl %ecx
jmp *%ecx
# Switch sections in the middle of a function
.section .text.foo, "ax"
.globl foo
.align 32, 0x90
.type foo,@function
# CHECK-LABEL: foo:
foo:
inc %eax
tmp3:
.rodata
.type obj,@object
.comm obj,4,4
.section .text.foo
inc %eax
# CHECK: tmp3:
# CHECK-NEXT: 1: incl

2
test/MC/X86/AlignedBundling/long-nop-pad.s
View File

@ -14,7 +14,7 @@ foo:
# To align this group to a bundle end, we need a 15-byte NOP and a 12-byte NOP.
# CHECK: 0: nop
# CHECK-NEXT: f: nop
# CHECK-NEXT: 1b: callq
# CHECK: 1b: callq
# This push instruction is 1 byte long
.bundle_lock align_to_end