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[RFC] Use pre-compiled headers to speed up LLVM build (~1.5-2x) #173868

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[RFC] Use pre-compiled headers to speed up LLVM build (~1.5-2x) #173868

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4160076
[LLD][MachO][NFC] Rename Reloc to Relocation
aengelke Jan 12, 2026
d776a7e
[CMake][LLVM] Add PCH infrastructure and LLVMSupport PCH
aengelke Jan 7, 2026
2d1b3db
[CMake][IR] Add PCH
aengelke Jan 7, 2026
8f7cb7f
[CMake][CodeGen] Add PCH
aengelke Jan 7, 2026
70c0593
[CMake][AST] Add PCH
aengelke Jan 7, 2026
2d7a7d7
[CMake][CodeGen] Add PCH for Clang CodeGen
aengelke Jan 7, 2026
8170761
pch for unittests
aengelke Dec 30, 2025
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32 changes: 32 additions & 0 deletions clang/include/clang/AST/pch.h
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@@ -0,0 +1,32 @@
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// Precompiled header for clangAST.
///
//===----------------------------------------------------------------------===//

#include "clang/AST/ASTContext.h"
#include "clang/AST/Attr.h"
#include "clang/AST/CanonicalType.h"
#include "clang/AST/Decl.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/DeclObjC.h"
#include "clang/AST/DeclOpenMP.h"
#include "clang/AST/DeclTemplate.h"
#include "clang/AST/DynamicRecursiveASTVisitor.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprCXX.h"
#include "clang/AST/ExprObjC.h"
#include "clang/AST/GlobalDecl.h"
#include "clang/AST/OpenMPClause.h"
#include "clang/AST/RecursiveASTVisitor.h"
#include "clang/AST/Stmt.h"
#include "clang/AST/StmtOpenMP.h"
#include "clang/AST/StmtVisitor.h"
#include "clang/AST/Type.h"
#include "llvm/Support/pch.h"
3 changes: 3 additions & 0 deletions clang/lib/AST/CMakeLists.txt
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Expand Up @@ -136,6 +136,9 @@ add_clang_library(clangAST
VTableBuilder.cpp
VTTBuilder.cpp

PRECOMPILE_HEADERS
[["clang/AST/pch.h"]]

LINK_LIBS
clangBasic
clangLex
Expand Down
5 changes: 4 additions & 1 deletion clang/lib/CodeGen/CGLoopInfo.cpp
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Expand Up @@ -18,9 +18,11 @@
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Metadata.h"
#include <optional>
using namespace clang::CodeGen;
using namespace llvm;

// No using namespace to avoid collision with llvm::LoopInfo.
namespace clang::CodeGen {

MDNode *
LoopInfo::createFollowupMetadata(const char *FollowupName,
ArrayRef<llvm::Metadata *> LoopProperties) {
Expand Down Expand Up @@ -863,3 +865,4 @@ void LoopInfoStack::InsertHelper(Instruction *I) const {
return;
}
}
} // end namespace clang::CodeGen
4 changes: 4 additions & 0 deletions clang/lib/CodeGen/CMakeLists.txt
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Expand Up @@ -158,6 +158,10 @@ add_clang_library(clangCodeGen
TrapReasonBuilder.cpp
VarBypassDetector.cpp

DISABLE_PCH_REUSE # PCH contains private headers
PRECOMPILE_HEADERS
[["pch.h"]]

DEPENDS
vt_gen
intrinsics_gen
Expand Down
22 changes: 22 additions & 0 deletions clang/lib/CodeGen/pch.h
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@@ -0,0 +1,22 @@
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// Precompiled header for clangCodeGen. Uses private headers.
///
//===----------------------------------------------------------------------===//

#include "ABIInfoImpl.h"
#include "Address.h"
#include "CGBuilder.h"
#include "CGCXXABI.h"
#include "CGValue.h"
#include "CodeGenFunction.h"
#include "CodeGenModule.h"
#include "clang/AST/pch.h"
#include "llvm/IR/pch.h"
#include "llvm/Support/pch.h"
4 changes: 2 additions & 2 deletions clang/lib/Frontend/TextDiagnostic.cpp
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Expand Up @@ -37,7 +37,7 @@ static constexpr raw_ostream::Colors SavedColor = raw_ostream::SAVEDCOLOR;
// is already taken for 'note'. Green is already used to underline
// source ranges. White and black are bad because of the usual
// terminal backgrounds. Which leaves us only with TWO options.
static constexpr raw_ostream::Colors CommentColor = raw_ostream::YELLOW;
static constexpr raw_ostream::Colors TCommentColor = raw_ostream::YELLOW;
static constexpr raw_ostream::Colors LiteralColor = raw_ostream::GREEN;
static constexpr raw_ostream::Colors KeywordColor = raw_ostream::BLUE;

Expand Down Expand Up @@ -1269,7 +1269,7 @@ highlightLines(StringRef FileData, unsigned StartLineNumber,
Vec.emplace_back(Start, Start + Length, LiteralColor);
} else {
assert(T.is(tok::comment));
Vec.emplace_back(Start, Start + Length, CommentColor);
Vec.emplace_back(Start, Start + Length, TCommentColor);
}
};

Expand Down
2 changes: 2 additions & 0 deletions clang/tools/c-index-test/CMakeLists.txt
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Expand Up @@ -5,6 +5,8 @@ set(LLVM_LINK_COMPONENTS
add_clang_executable(c-index-test
c-index-test.c
core_main.cpp

DISABLE_PCH_REUSE # Prevent CMake error with C source files.
)

if(NOT MSVC)
Expand Down
2 changes: 2 additions & 0 deletions clang/tools/clang-fuzzer/dictionary/CMakeLists.txt
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@@ -1,5 +1,7 @@
set(CMAKE_CXX_FLAGS ${CXX_FLAGS_NOFUZZ})
add_clang_executable(clang-fuzzer-dictionary
dictionary.c

DISABLE_PCH_REUSE # no C++ files, prevent CMake error.
)

1 change: 0 additions & 1 deletion clang/unittests/Basic/CharInfoTest.cpp
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Expand Up @@ -9,7 +9,6 @@
#include "clang/Basic/CharInfo.h"
#include "gtest/gtest.h"

using namespace llvm;
using namespace clang;

// Check that the CharInfo table has been constructed reasonably.
Expand Down
2 changes: 1 addition & 1 deletion clang/unittests/Interpreter/ExceptionTests/CMakeLists.txt
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@@ -1,7 +1,7 @@
# The interpreter can throw an exception from user input. The test binary needs
# to be compiled with exception support to catch the thrown exception.
set(LLVM_REQUIRES_EH ON)
set(LLVM_REQUIRES_RTTI ON)
set(LLVM_REQUIRES_RTTI OFF)

add_distinct_clang_unittest(ClangReplInterpreterExceptionTests
InterpreterExceptionTest.cpp
Expand Down
10 changes: 0 additions & 10 deletions flang/CMakeLists.txt
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Expand Up @@ -441,11 +441,6 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
if (BUILD_SHARED_LIBS AND NOT (CMAKE_CXX_COMPILER_ID MATCHES "Clang"))
set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fno-semantic-interposition")
endif()

# GCC requires this flag in order for precompiled headers to work with ccache
if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpch-preprocess")
endif()
endif()

# Clang on Darwin enables non-POSIX extensions by default, which allows the
Expand All @@ -456,11 +451,6 @@ if (APPLE)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
endif()

# Clang requires this flag in order for precompiled headers to work with ccache
if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -fno-pch-timestamp")
endif()

list(REMOVE_DUPLICATES CMAKE_CXX_FLAGS)

# Determine HOST_LINK_VERSION on Darwin.
Expand Down
18 changes: 9 additions & 9 deletions flang/lib/Evaluate/CMakeLists.txt
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Expand Up @@ -66,15 +66,8 @@ add_flang_library(FortranEvaluate
${LIBPGMATH}
${QUADMATHLIB}

LINK_COMPONENTS
Support

DEPENDS
acc_gen
omp_gen
)

target_precompile_headers(FortranEvaluate PRIVATE
DISABLE_PCH_REUSE
PRECOMPILE_HEADERS
[["flang/Evaluate/common.h"]]
[["flang/Evaluate/call.h"]]
[["flang/Evaluate/traverse.h"]]
Expand All @@ -86,4 +79,11 @@ target_precompile_headers(FortranEvaluate PRIVATE
[["flang/Evaluate/integer.h"]]
[["flang/Evaluate/expression.h"]]
[["flang/Evaluate/tools.h"]]

LINK_COMPONENTS
Support

DEPENDS
acc_gen
omp_gen
)
16 changes: 8 additions & 8 deletions flang/lib/Frontend/CMakeLists.txt
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Expand Up @@ -13,6 +13,14 @@ add_flang_library(flangFrontend
TextDiagnosticBuffer.cpp
TextDiagnostic.cpp

DISABLE_PCH_REUSE
PRECOMPILE_HEADERS
[["flang/Parser/parsing.h"]]
[["flang/Parser/parse-tree.h"]]
[["flang/Parser/dump-parse-tree.h"]]
[["flang/Lower/PFTBuilder.h"]]
[["flang/Lower/Bridge.h"]]

DEPENDS
CUFDialect
FIRDialect
Expand Down Expand Up @@ -78,11 +86,3 @@ add_flang_library(flangFrontend
clangBasic
clangOptions
)

target_precompile_headers(flangFrontend PRIVATE
[["flang/Parser/parsing.h"]]
[["flang/Parser/parse-tree.h"]]
[["flang/Parser/dump-parse-tree.h"]]
[["flang/Lower/PFTBuilder.h"]]
[["flang/Lower/Bridge.h"]]
)
22 changes: 11 additions & 11 deletions flang/lib/Lower/CMakeLists.txt
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Expand Up @@ -38,6 +38,17 @@ add_flang_library(FortranLower
Support/Utils.cpp
SymbolMap.cpp
VectorSubscripts.cpp

DISABLE_PCH_REUSE
PRECOMPILE_HEADERS
[["flang/Lower/ConvertExpr.h"]]
[["flang/Lower/SymbolMap.h"]]
[["flang/Lower/AbstractConverter.h"]]
[["flang/Lower/IterationSpace.h"]]
[["flang/Lower/CallInterface.h"]]
[["flang/Lower/BoxAnalyzer.h"]]
[["flang/Lower/PFTBuilder.h"]]
[["flang/Lower/DirectivesCommon.h"]]

DEPENDS
CUFAttrs
Expand Down Expand Up @@ -79,14 +90,3 @@ add_flang_library(FortranLower
MLIRLLVMDialect
MLIRSCFToControlFlow
)

target_precompile_headers(FortranLower PRIVATE
[["flang/Lower/ConvertExpr.h"]]
[["flang/Lower/SymbolMap.h"]]
[["flang/Lower/AbstractConverter.h"]]
[["flang/Lower/IterationSpace.h"]]
[["flang/Lower/CallInterface.h"]]
[["flang/Lower/BoxAnalyzer.h"]]
[["flang/Lower/PFTBuilder.h"]]
[["flang/Lower/DirectivesCommon.h"]]
)
16 changes: 8 additions & 8 deletions flang/lib/Parser/CMakeLists.txt
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Expand Up @@ -25,6 +25,14 @@ add_flang_library(FortranParser
unparse.cpp
user-state.cpp

DISABLE_PCH_REUSE
PRECOMPILE_HEADERS
[["flang/Parser/parsing.h"]]
[["flang/Parser/parse-tree.h"]]
[["flang/Parser/provenance.h"]]
[["flang/Parser/message.h"]]
[["flang/Parser/parse-tree-visitor.h"]]

LINK_LIBS
FortranSupport

Expand All @@ -37,11 +45,3 @@ add_flang_library(FortranParser
omp_gen
acc_gen
)

target_precompile_headers(FortranParser PRIVATE
[["flang/Parser/parsing.h"]]
[["flang/Parser/parse-tree.h"]]
[["flang/Parser/provenance.h"]]
[["flang/Parser/message.h"]]
[["flang/Parser/parse-tree-visitor.h"]]
)
18 changes: 9 additions & 9 deletions flang/lib/Semantics/CMakeLists.txt
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Expand Up @@ -53,6 +53,15 @@ add_flang_library(FortranSemantics
type.cpp
unparse-with-symbols.cpp

DISABLE_PCH_REUSE
PRECOMPILE_HEADERS
[["flang/Semantics/semantics.h"]]
[["flang/Semantics/type.h"]]
[["flang/Semantics/openmp-modifiers.h"]]
[["flang/Semantics/expression.h"]]
[["flang/Semantics/tools.h"]]
[["flang/Semantics/symbol.h"]]

DEPENDS
acc_gen
omp_gen
Expand All @@ -68,12 +77,3 @@ add_flang_library(FortranSemantics
FrontendOpenACC
TargetParser
)

target_precompile_headers(FortranSemantics PRIVATE
[["flang/Semantics/semantics.h"]]
[["flang/Semantics/type.h"]]
[["flang/Semantics/openmp-modifiers.h"]]
[["flang/Semantics/expression.h"]]
[["flang/Semantics/tools.h"]]
[["flang/Semantics/symbol.h"]]
)
10 changes: 5 additions & 5 deletions lld/MachO/Arch/ARM64Common.cpp
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Expand Up @@ -38,7 +38,7 @@ int64_t ARM64Common::getEmbeddedAddend(MemoryBufferRef mb, uint64_t offset,
}
}

static void writeValue(uint8_t *loc, const Reloc &r, uint64_t value) {
static void writeValue(uint8_t *loc, const Relocation &r, uint64_t value) {
switch (r.length) {
case 2:
checkInt(loc, r, value, 32);
Expand All @@ -57,7 +57,7 @@ static void writeValue(uint8_t *loc, const Reloc &r, uint64_t value) {
// instruction has opcode & register-operand bits set, with immediate
// operands zeroed. We read it from text, OR-in the immediate
// operands, then write-back the completed instruction.
void ARM64Common::relocateOne(uint8_t *loc, const Reloc &r, uint64_t value,
void ARM64Common::relocateOne(uint8_t *loc, const Relocation &r, uint64_t value,
uint64_t pc) const {
auto loc32 = reinterpret_cast<uint32_t *>(loc);
uint32_t base = ((r.length == 2) ? read32le(loc) : 0);
Expand Down Expand Up @@ -110,7 +110,7 @@ void ARM64Common::relaxGotLoad(uint8_t *loc, uint8_t type) const {
write32le(loc, instruction);
}

void ARM64Common::handleDtraceReloc(const Symbol *sym, const Reloc &r,
void ARM64Common::handleDtraceReloc(const Symbol *sym, const Relocation &r,
uint8_t *loc) const {
assert(r.type == ARM64_RELOC_BRANCH26);

Expand Down Expand Up @@ -138,8 +138,8 @@ static void reportUnalignedLdrStr(Twine loc, uint64_t va, int align,
"-byte aligned");
}

void macho::reportUnalignedLdrStr(void *loc, const lld::macho::Reloc &r,
uint64_t va, int align) {
void macho::reportUnalignedLdrStr(void *loc, const Relocation &r, uint64_t va,
int align) {
uint64_t off = reinterpret_cast<const uint8_t *>(loc) - in.bufferStart;
const InputSection *isec = offsetToInputSection(&off);
std::string locStr = isec ? isec->getLocation(off) : "(invalid location)";
Expand Down
11 changes: 6 additions & 5 deletions lld/MachO/Arch/ARM64Common.h
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Expand Up @@ -23,13 +23,13 @@ struct ARM64Common : TargetInfo {

int64_t getEmbeddedAddend(MemoryBufferRef, uint64_t offset,
const llvm::MachO::relocation_info) const override;
void relocateOne(uint8_t *loc, const Reloc &, uint64_t va,
void relocateOne(uint8_t *loc, const Relocation &, uint64_t va,
uint64_t pc) const override;

void relaxGotLoad(uint8_t *loc, uint8_t type) const override;
uint64_t getPageSize() const override { return 16 * 1024; }

void handleDtraceReloc(const Symbol *sym, const Reloc &r,
void handleDtraceReloc(const Symbol *sym, const Relocation &r,
uint8_t *loc) const override;
};

Expand All @@ -42,7 +42,7 @@ inline uint64_t bitField(uint64_t value, int right, int width, int left) {
// | | imm26 |
// +-----------+---------------------------------------------------+

inline void encodeBranch26(uint32_t *loc, const Reloc &r, uint32_t base,
inline void encodeBranch26(uint32_t *loc, const Relocation &r, uint32_t base,
uint64_t va) {
checkInt(loc, r, va, 28);
// Since branch destinations are 4-byte aligned, the 2 least-
Expand All @@ -61,7 +61,7 @@ inline void encodeBranch26(uint32_t *loc, SymbolDiagnostic d, uint32_t base,
// | |ilo| | immhi | |
// +-+---+---------+-------------------------------------+---------+

inline void encodePage21(uint32_t *loc, const Reloc &r, uint32_t base,
inline void encodePage21(uint32_t *loc, const Relocation &r, uint32_t base,
uint64_t va) {
checkInt(loc, r, va, 35);
llvm::support::endian::write32le(loc, base | bitField(va, 12, 2, 29) |
Expand All @@ -75,7 +75,8 @@ inline void encodePage21(uint32_t *loc, SymbolDiagnostic d, uint32_t base,
bitField(va, 14, 19, 5));
}

void reportUnalignedLdrStr(void *loc, const Reloc &, uint64_t va, int align);
void reportUnalignedLdrStr(void *loc, const Relocation &, uint64_t va,
int align);
void reportUnalignedLdrStr(void *loc, SymbolDiagnostic, uint64_t va, int align);

// 21 10
Expand Down
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