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Driver, CodeGen: Handle -falign-functions, -fno-align-functions and -falign-functions=1 correctly. #149445

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Driver, CodeGen: Handle -falign-functions, -fno-align-functions and -falign-functions=1 correctly. #149445

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dc3d07d
[𝘀𝗽𝗿] initial version
pcc Jul 18, 2025
99a4bcd
Omit flag if 0
pcc Jul 18, 2025
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4 changes: 2 additions & 2 deletions clang/include/clang/Driver/CommonArgs.h
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Original file line number Diff line number Diff line change
Expand Up @@ -85,8 +85,8 @@ const char *RelocationModelName(llvm::Reloc::Model Model);
std::tuple<llvm::Reloc::Model, unsigned, bool>
ParsePICArgs(const ToolChain &ToolChain, const llvm::opt::ArgList &Args);

unsigned ParseFunctionAlignment(const ToolChain &TC,
const llvm::opt::ArgList &Args);
llvm::MaybeAlign ParseFunctionAlignment(const ToolChain &TC,
const llvm::opt::ArgList &Args);

void addDebugInfoKind(llvm::opt::ArgStringList &CmdArgs,
llvm::codegenoptions::DebugInfoKind DebugInfoKind);
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8 changes: 5 additions & 3 deletions clang/lib/CodeGen/CodeGenModule.cpp
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Expand Up @@ -2760,9 +2760,11 @@ void CodeGenModule::SetLLVMFunctionAttributesForDefinition(const Decl *D,
if (alignment)
F->setAlignment(llvm::Align(alignment));

if (!D->hasAttr<AlignedAttr>())
if (LangOpts.FunctionAlignment)
F->setAlignment(llvm::Align(1ull << LangOpts.FunctionAlignment));
if (!D->hasAttr<AlignedAttr>()) {
llvm::MaybeAlign Align = llvm::decodeMaybeAlign(LangOpts.FunctionAlignment);
if (Align)
F->setAlignment(*Align);
}

// Some C++ ABIs require 2-byte alignment for member functions, in order to
// reserve a bit for differentiating between virtual and non-virtual member
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11 changes: 5 additions & 6 deletions clang/lib/Driver/ToolChains/Clang.cpp
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Expand Up @@ -40,6 +40,7 @@
#include "llvm/Frontend/Debug/Options.h"
#include "llvm/Object/ObjectFile.h"
#include "llvm/Option/ArgList.h"
#include "llvm/Support/Alignment.h"
#include "llvm/Support/CodeGen.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Compression.h"
Expand Down Expand Up @@ -5516,12 +5517,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,

CheckCodeGenerationOptions(D, Args);

unsigned FunctionAlignment = ParseFunctionAlignment(TC, Args);
assert(FunctionAlignment <= 31 && "function alignment will be truncated!");
if (FunctionAlignment) {
CmdArgs.push_back("-function-alignment");
CmdArgs.push_back(Args.MakeArgString(std::to_string(FunctionAlignment)));
}
llvm::MaybeAlign FunctionAlignment = ParseFunctionAlignment(TC, Args);
CmdArgs.push_back("-function-alignment");
CmdArgs.push_back(
Args.MakeArgString(std::to_string(llvm::encode(FunctionAlignment))));
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Why add the arg in the !FunctionAlignment case?

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I guess it isn't strictly necessary, but it ensures that CodeGen will always receive a value that was encoded by llvm::encode. That being said it seems unlikely that we will actually change the meaning of the 0 encoding.

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I'd just omit it and reduce flag spam

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Done


// We support -falign-loops=N where N is a power of 2. GCC supports more
// forms.
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20 changes: 12 additions & 8 deletions clang/lib/Driver/ToolChains/CommonArgs.cpp
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Expand Up @@ -2120,29 +2120,33 @@ tools::ParsePICArgs(const ToolChain &ToolChain, const ArgList &Args) {
// [0, 65536]. If the value is not a power-of-two, it will be rounded up to
// the nearest power-of-two.
//
// If we return `0`, the frontend will default to the backend's preferred
// alignment.
// If we return `MaybeAlign()`, the frontend will default to the backend's
// preferred alignment.
//
// NOTE: icc only allows values between [0, 4096]. icc uses `-falign-functions`
// to mean `-falign-functions=16`. GCC defaults to the backend's preferred
// alignment. For unaligned functions, we default to the backend's preferred
// alignment.
unsigned tools::ParseFunctionAlignment(const ToolChain &TC,
const ArgList &Args) {
llvm::MaybeAlign tools::ParseFunctionAlignment(const ToolChain &TC,
const ArgList &Args) {
const Arg *A = Args.getLastArg(options::OPT_falign_functions,
options::OPT_falign_functions_EQ,
options::OPT_fno_align_functions);
if (!A || A->getOption().matches(options::OPT_fno_align_functions))
return 0;
if (!A)
return llvm::MaybeAlign();

if (A->getOption().matches(options::OPT_fno_align_functions))
return llvm::Align(1);

if (A->getOption().matches(options::OPT_falign_functions))
return 0;
return llvm::MaybeAlign();

unsigned Value = 0;
if (StringRef(A->getValue()).getAsInteger(10, Value) || Value > 65536)
TC.getDriver().Diag(diag::err_drv_invalid_int_value)
<< A->getAsString(Args) << A->getValue();
return Value ? llvm::Log2_32_Ceil(std::min(Value, 65536u)) : Value;
return Value ? llvm::Align(1 << llvm::Log2_32_Ceil(std::min(Value, 65536u)))
: llvm::MaybeAlign();
}

void tools::addDebugInfoKind(
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10 changes: 7 additions & 3 deletions clang/test/CodeGen/function-alignment.c
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@@ -1,13 +1,17 @@
// RUN: %clang_cc1 -emit-llvm %s -o - | FileCheck %s -check-prefix CHECK-NONE
// RUN: %clang_cc1 -emit-llvm -function-alignment 4 %s -o - | FileCheck %s -check-prefix CHECK-16
// RUN: %clang_cc1 -emit-llvm -function-alignment 5 %s -o - | FileCheck %s -check-prefix CHECK-32
// RUN: %clang_cc1 -emit-llvm -function-alignment 0 %s -o - | FileCheck %s -check-prefix CHECK-NONE
// RUN: %clang_cc1 -emit-llvm -function-alignment 1 %s -o - | FileCheck %s -check-prefix CHECK-1
// RUN: %clang_cc1 -emit-llvm -function-alignment 5 %s -o - | FileCheck %s -check-prefix CHECK-16
// RUN: %clang_cc1 -emit-llvm -function-alignment 6 %s -o - | FileCheck %s -check-prefix CHECK-32

void f(void) {}
void __attribute__((__aligned__(64))) g(void) {}

// CHECK-NONE-NOT: define {{(dso_local )?}}void @f() #0 align
// CHECK-NONE: define {{(dso_local )?}}void @g() #0 align 64

// CHECK-1: define {{(dso_local )?}}void @f() #0 align 1
// CHECK-1: define {{(dso_local )?}}void @g() #0 align 64

// CHECK-16: define {{(dso_local )?}}void @f() #0 align 16
// CHECK-16: define {{(dso_local )?}}void @g() #0 align 64

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14 changes: 7 additions & 7 deletions clang/test/Driver/function-alignment.c
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@@ -1,17 +1,17 @@
// RUN: %clang -### %s 2>&1 | FileCheck %s -check-prefix CHECK-0
// RUN: %clang -### -falign-functions %s 2>&1 | FileCheck %s -check-prefix CHECK-1
// RUN: %clang -### -falign-functions %s 2>&1 | FileCheck %s -check-prefix CHECK-0
// RUN: %clang -### -fno-align-functions %s 2>&1 | FileCheck %s -check-prefix CHECK-1
// RUN: %clang -### -falign-functions=1 %s 2>&1 | FileCheck %s -check-prefix CHECK-1
// RUN: %clang -### -falign-functions=2 %s 2>&1 | FileCheck %s -check-prefix CHECK-2
// RUN: %clang -### -falign-functions=3 %s 2>&1 | FileCheck %s -check-prefix CHECK-3
// RUN: %clang -### -falign-functions=4 %s 2>&1 | FileCheck %s -check-prefix CHECK-4
// RUN: %clang -### -falign-functions=4 %s 2>&1 | FileCheck %s -check-prefix CHECK-3
// RUN: not %clang -### -falign-functions=65537 %s 2>&1 | FileCheck %s -check-prefix CHECK-ERR-65537
// RUN: not %clang -### -falign-functions=a %s 2>&1 | FileCheck %s -check-prefix CHECK-ERR-A

// CHECK-0-NOT: "-function-alignment"
// CHECK-1-NOT: "-function-alignment"
// CHECK-2: "-function-alignment" "1"
// CHECK-3: "-function-alignment" "2"
// CHECK-4: "-function-alignment" "2"
// CHECK-0: "-function-alignment" "0"
// CHECK-1: "-function-alignment" "1"
// CHECK-2: "-function-alignment" "2"
// CHECK-3: "-function-alignment" "3"
// CHECK-ERR-65537: error: invalid integral value '65537' in '-falign-functions=65537'
// CHECK-ERR-A: error: invalid integral value 'a' in '-falign-functions=a'

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