[llvm] MC: Avoid using MCSubtargetInfo for fragment reuse determination (PR #149471)
Fangrui Song via llvm-commits
llvm-commits at lists.llvm.org
Fri Jul 18 01:20:25 PDT 2025
https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/149471
Reduce unnecessary overhead introduced in
https://reviews.llvm.org/D44928 . That change passed STI to both FT_Data
and FT_Relaxable fragments, but STI is only necessary for FT_Relaxable.
The use of STI in FT_Data was added for:
* Bundle alignment mode, which has been removed (#148781).
* ARM, which inappropriately uses STI in `ARMAsmBackend::applyFixup` due
to tech debt, unlike other targets.
To ensure safety, `copySTI` now starts a new fragment to prevent mixed
STI values.
>From 2efea545f37c57f52037168ec15aa4596ba8fcf2 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Fri, 18 Jul 2025 01:20:16 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.5-bogner
---
llvm/include/llvm/MC/MCObjectStreamer.h | 13 +------------
llvm/include/llvm/MC/MCSection.h | 1 +
llvm/include/llvm/MC/MCStreamer.h | 4 +++-
llvm/lib/MC/MCObjectStreamer.cpp | 16 ++++------------
llvm/lib/MC/MCParser/MCTargetAsmParser.cpp | 4 ++++
llvm/lib/MC/MCStreamer.cpp | 13 +++++++++++++
6 files changed, 26 insertions(+), 25 deletions(-)
diff --git a/llvm/include/llvm/MC/MCObjectStreamer.h b/llvm/include/llvm/MC/MCObjectStreamer.h
index a55fd4a14675f..319e131999d48 100644
--- a/llvm/include/llvm/MC/MCObjectStreamer.h
+++ b/llvm/include/llvm/MC/MCObjectStreamer.h
@@ -73,20 +73,9 @@ class MCObjectStreamer : public MCStreamer {
MCSymbol *emitCFILabel() override;
void emitCFISections(bool EH, bool Debug) override;
- void insert(MCFragment *F) {
- auto *Sec = CurFrag->getParent();
- F->setParent(Sec);
- F->setLayoutOrder(CurFrag->getLayoutOrder() + 1);
- CurFrag->Next = F;
- CurFrag = F;
- Sec->curFragList()->Tail = F;
- }
-
/// Get a data fragment to write into, creating a new one if the current
/// fragment is not FT_Data.
- /// Optionally a \p STI can be passed in so that a new fragment is created
- /// if the Subtarget differs from the current fragment.
- MCFragment *getOrCreateDataFragment(const MCSubtargetInfo *STI = nullptr);
+ MCFragment *getOrCreateDataFragment();
protected:
bool changeSectionImpl(MCSection *Section, uint32_t Subsection);
diff --git a/llvm/include/llvm/MC/MCSection.h b/llvm/include/llvm/MC/MCSection.h
index 296fdd8af0d14..313071ec75033 100644
--- a/llvm/include/llvm/MC/MCSection.h
+++ b/llvm/include/llvm/MC/MCSection.h
@@ -188,6 +188,7 @@ class LLVM_ABI MCSection {
// destructors.
class MCFragment {
friend class MCAssembler;
+ friend class MCStreamer;
friend class MCObjectStreamer;
friend class MCSection;
diff --git a/llvm/include/llvm/MC/MCStreamer.h b/llvm/include/llvm/MC/MCStreamer.h
index b3a9aabd6ece5..4b91dbc794682 100644
--- a/llvm/include/llvm/MC/MCStreamer.h
+++ b/llvm/include/llvm/MC/MCStreamer.h
@@ -429,7 +429,6 @@ class LLVM_ABI MCStreamer {
CurFrag->getParent() == getCurrentSection().first);
return CurFrag;
}
-
/// Save the current and previous section on the section stack.
void pushSection() {
SectionStack.push_back(
@@ -457,6 +456,9 @@ class LLVM_ABI MCStreamer {
MCSymbol *endSection(MCSection *Section);
+ void insert(MCFragment *F);
+ void newFragment();
+
/// Returns the mnemonic for \p MI, if the streamer has access to a
/// instruction printer and returns an empty string otherwise.
virtual StringRef getMnemonic(const MCInst &MI) const { return ""; }
diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 67433f2b265e5..8a3e8efee0b36 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -106,26 +106,18 @@ void MCObjectStreamer::emitFrames(MCAsmBackend *MAB) {
MCDwarfFrameEmitter::Emit(*this, MAB, false);
}
-static bool canReuseDataFragment(const MCFragment &F,
- const MCAssembler &Assembler,
- const MCSubtargetInfo *STI) {
+static bool canReuseDataFragment(const MCFragment &F) {
if (!F.hasInstructions())
return true;
// Do not add data after a linker-relaxable instruction. The difference
// between a new label and a label at or before the linker-relaxable
// instruction cannot be resolved at assemble-time.
- if (F.isLinkerRelaxable())
- return false;
- // If the subtarget is changed mid fragment we start a new fragment to record
- // the new STI.
- return !STI || F.getSubtargetInfo() == STI;
+ return !F.isLinkerRelaxable();
}
-MCFragment *
-MCObjectStreamer::getOrCreateDataFragment(const MCSubtargetInfo *STI) {
+MCFragment *MCObjectStreamer::getOrCreateDataFragment() {
auto *F = getCurrentFragment();
- if (F->getKind() != MCFragment::FT_Data ||
- !canReuseDataFragment(*F, *Assembler, STI)) {
+ if (F->getKind() != MCFragment::FT_Data || !canReuseDataFragment(*F)) {
F = getContext().allocFragment<MCFragment>();
insert(F);
}
diff --git a/llvm/lib/MC/MCParser/MCTargetAsmParser.cpp b/llvm/lib/MC/MCParser/MCTargetAsmParser.cpp
index 665d92eb9a21c..44498f21a9060 100644
--- a/llvm/lib/MC/MCParser/MCTargetAsmParser.cpp
+++ b/llvm/lib/MC/MCParser/MCTargetAsmParser.cpp
@@ -9,6 +9,7 @@
#include "llvm/MC/MCParser/MCTargetAsmParser.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCRegister.h"
+#include "llvm/MC/MCStreamer.h"
using namespace llvm;
@@ -22,6 +23,9 @@ MCTargetAsmParser::~MCTargetAsmParser() = default;
MCSubtargetInfo &MCTargetAsmParser::copySTI() {
MCSubtargetInfo &STICopy = getContext().getSubtargetCopy(getSTI());
STI = &STICopy;
+ // The returned STI will likely be modified. Create a new fragment to avoid
+ // mixed STI values within a fragment.
+ getStreamer().newFragment();
return STICopy;
}
diff --git a/llvm/lib/MC/MCStreamer.cpp b/llvm/lib/MC/MCStreamer.cpp
index d814ab8880500..c3ecf8fc717f5 100644
--- a/llvm/lib/MC/MCStreamer.cpp
+++ b/llvm/lib/MC/MCStreamer.cpp
@@ -1404,6 +1404,19 @@ MCSymbol *MCStreamer::endSection(MCSection *Section) {
return Sym;
}
+void MCStreamer::insert(MCFragment *F) {
+ auto *Sec = CurFrag->getParent();
+ F->setParent(Sec);
+ F->setLayoutOrder(CurFrag->getLayoutOrder() + 1);
+ CurFrag->Next = F;
+ CurFrag = F;
+ Sec->curFragList()->Tail = F;
+}
+
+void MCStreamer::newFragment() {
+ insert(getContext().allocFragment<MCFragment>());
+}
+
static VersionTuple
targetVersionOrMinimumSupportedOSVersion(const Triple &Target,
VersionTuple TargetVersion) {
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