From 583dac827ca7daea75c6e92c4ebf6b2296118886 Mon Sep 17 00:00:00 2001
From: "Duncan P. N. Exon Smith" <dexonsmith@apple.com>
Date: Thu, 21 May 2015 02:41:23 +0000
Subject: [PATCH] AsmPrinter: Compute absolute label difference directly
Create a low-overhead path for `EmitLabelDifference()` that emits a
emits an absolute number when (1) the output is an object stream and (2)
the two symbols are in the same data fragment.
This drops memory usage on Mach-O from 975 MB down to 919 MB (5.8%).
The only call is when `!doesDwarfUseRelocationsAcrossSections()` --
i.e., on Mach-O -- since otherwise an absolute offset from the start of
the section needs a relocation. (`EmitLabelDifference()` is cheaper on
ELF anyway, since it creates 1 fewer temp symbol, and it gets called far
less often. It's not clear to me if this is even a bottleneck there.)
(I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
see r236629 for details.)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237876 91177308-0d34-0410-b5e6-96231b3b80d8
---
include/llvm/MC/MCObjectStreamer.h | 11 +++++++++++
include/llvm/MC/MCStreamer.h | 9 +++++++++
lib/CodeGen/AsmPrinter/AsmPrinter.cpp | 4 ++++
lib/MC/MCObjectStreamer.cpp | 23 +++++++++++++++++++++++
4 files changed, 47 insertions(+)
diff --git a/include/llvm/MC/MCObjectStreamer.h b/include/llvm/MC/MCObjectStreamer.h
index 8499560d2d7..1a1222562f1 100644
--- a/include/llvm/MC/MCObjectStreamer.h
+++ b/include/llvm/MC/MCObjectStreamer.h
@@ -136,6 +136,17 @@ public:
void EmitZeros(uint64_t NumBytes) override;
void FinishImpl() override;
+ /// Emit the absolute difference between two symbols if possible.
+ ///
+ /// Emit the absolute difference between \c Hi and \c Lo, as long as we can
+ /// compute it. Currently, that requires that both symbols are in the same
+ /// data fragment. Otherwise, do nothing and return \c false.
+ ///
+ /// \pre Offset of \c Hi is greater than the offset \c Lo.
+ /// \return true on success.
+ bool emitAbsoluteSymbolDiff(const MCSymbol *Hi, const MCSymbol *Lo,
+ unsigned Size) override;
+
bool mayHaveInstructions() const override {
return getCurrentSectionData()->hasInstructions();
}
diff --git a/include/llvm/MC/MCStreamer.h b/include/llvm/MC/MCStreamer.h
index 9107df62933..3dec9491a26 100644
--- a/include/llvm/MC/MCStreamer.h
+++ b/include/llvm/MC/MCStreamer.h
@@ -652,6 +652,15 @@ public:
unsigned Isa, unsigned Discriminator,
StringRef FileName);
+ /// Emit the absolute difference between two symbols if possible.
+ ///
+ /// \pre Offset of \c Hi is greater than the offset \c Lo.
+ /// \return true on success.
+ virtual bool emitAbsoluteSymbolDiff(const MCSymbol *Hi, const MCSymbol *Lo,
+ unsigned Size) {
+ return false;
+ }
+
virtual MCSymbol *getDwarfLineTableSymbol(unsigned CUID);
virtual void EmitCFISections(bool EH, bool Debug);
void EmitCFIStartProc(bool IsSimple);
diff --git a/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 5af083536d7..17bbf01c43f 100644
--- a/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1591,6 +1591,10 @@ void AsmPrinter::EmitInt32(int Value) const {
/// .set if it avoids relocations.
void AsmPrinter::EmitLabelDifference(const MCSymbol *Hi, const MCSymbol *Lo,
unsigned Size) const {
+ if (!MAI->doesDwarfUseRelocationsAcrossSections())
+ if (OutStreamer->emitAbsoluteSymbolDiff(Hi, Lo, Size))
+ return;
+
// Get the Hi-Lo expression.
const MCExpr *Diff =
MCBinaryExpr::CreateSub(MCSymbolRefExpr::Create(Hi, OutContext),
diff --git a/lib/MC/MCObjectStreamer.cpp b/lib/MC/MCObjectStreamer.cpp
index d82dd15df2b..836b1ced5c5 100644
--- a/lib/MC/MCObjectStreamer.cpp
+++ b/lib/MC/MCObjectStreamer.cpp
@@ -53,6 +53,29 @@ void MCObjectStreamer::flushPendingLabels(MCFragment *F, uint64_t FOffset) {
}
}
+bool MCObjectStreamer::emitAbsoluteSymbolDiff(const MCSymbol *Hi,
+ const MCSymbol *Lo,
+ unsigned Size) {
+ // Must have symbol data.
+ if (!Assembler->hasSymbolData(*Hi) || !Assembler->hasSymbolData(*Lo))
+ return false;
+ auto &HiD = Assembler->getSymbolData(*Hi);
+ auto &LoD = Assembler->getSymbolData(*Lo);
+
+ // Must both be assigned to the same (valid) fragment.
+ if (!HiD.getFragment() || HiD.getFragment() != LoD.getFragment())
+ return false;
+
+ // Must be a data fragment.
+ if (!isa<MCDataFragment>(HiD.getFragment()))
+ return false;
+
+ assert(HiD.getOffset() >= LoD.getOffset() &&
+ "Expected Hi to be greater than Lo");
+ EmitIntValue(HiD.getOffset() - LoD.getOffset(), Size);
+ return true;
+}
+
void MCObjectStreamer::reset() {
if (Assembler)
Assembler->reset();
--
2.34.1