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[clang-cl] Support the /HOTPATCH flag
This patch adds support for the MSVC /HOTPATCH flag: https://docs.microsoft.com/sv-se/cpp/build/reference/hotpatch-create-hotpatchable-image?view=msvc-170&viewFallbackFrom=vs-2019 The flag is translated to a new -fms-hotpatch flag, which in turn adds a 'patchable-function' attribute for each function in the TU. This is then picked up by the PatchableFunction pass which would generate a TargetOpcode::PATCHABLE_OP of minsize = 2 (which means the target instruction must resolve to at least two bytes). TargetOpcode::PATCHABLE_OP is only implemented for x86/x64. When targetting ARM/ARM64, /HOTPATCH isn't required (instructions are always 2/4 bytes and suitable for hotpatching). Additionally, when using /Z7, we generate a 'hot patchable' flag in the CodeView debug stream, in the S_COMPILE3 record. This flag is then picked up by LLD (or link.exe) and is used in conjunction with the linker /FUNCTIONPADMIN flag to generate extra space before each function, to accommodate for live patching long jumps. Please see: https://github.com/llvm/llvm-project/blob/d703b922961e0d02a5effdd4bfbb23ad50a3cc9f/lld/COFF/Writer.cpp#L1298 The outcome is that we can finally use Live++ or Recode along with clang-cl. NOTE: It seems that MSVC cl.exe always enables /HOTPATCH on x64 by default, although if we did the same I thought we might generate sub-optimal code (if this flag was active by default). Additionally, MSVC always generates a .debug$S section and a S_COMPILE3 record, which Clang doesn't do without /Z7. Therefore, the following MSVC command-line "cl /c file.cpp" would have to be written with Clang such as "clang-cl /c file.cpp /HOTPATCH /Z7" in order to obtain the same result. Depends on D43002, D80833 and D81301 for the full feature. Differential Revision: https://reviews.llvm.org/D116511
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‎clang/docs/ReleaseNotes.rst

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@@ -136,6 +136,16 @@ Windows Support
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or pass ``/permissive`` to disable C++ operator names altogether. See
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`PR42427 <https://llvm.org/pr42427>` for more info.
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- Add support for MSVC-compatible ``/hotpatch`` flag in clang-cl, and equivalent
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-cc1 flag ``-fms-hotpatch``. Along with the linker flag ``/functionpadmin``
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this creates executable images suitable for runtime code patching. This flag
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is only required for x86/x64 targets; ARM/ARM64 simply needs the linker
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``/functionpadmin``.
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With this addition, clang-cl can be used in live code patching scenarios,
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along with tools such as Live++ or Recode. Microsoft Edit and Continue isn't
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currently supported.
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C Language Changes in Clang
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---------------------------
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‎clang/include/clang/Basic/CodeGenOptions.def

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@@ -139,6 +139,9 @@ VALUE_CODEGENOPT(XRaySelectedFunctionGroup, 32, 0)
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VALUE_CODEGENOPT(PatchableFunctionEntryCount , 32, 0) ///< Number of NOPs at function entry
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VALUE_CODEGENOPT(PatchableFunctionEntryOffset , 32, 0)
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CODEGENOPT(HotPatch, 1, 0) ///< Supports the Microsoft /HOTPATCH flag and
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///< generates a 'patchable-function' attribute.
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CODEGENOPT(InstrumentForProfiling , 1, 0) ///< Set when -pg is enabled.
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CODEGENOPT(CallFEntry , 1, 0) ///< Set when -mfentry is enabled.
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CODEGENOPT(MNopMCount , 1, 0) ///< Set when -mnop-mcount is enabled.

‎clang/include/clang/Driver/Options.td

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@@ -2498,6 +2498,9 @@ defm pascal_strings : BoolFOption<"pascal-strings",
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def fpatchable_function_entry_EQ : Joined<["-"], "fpatchable-function-entry=">, Group<f_Group>, Flags<[CC1Option]>,
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MetaVarName<"<N,M>">, HelpText<"Generate M NOPs before function entry and N-M NOPs after function entry">,
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MarshallingInfoInt<CodeGenOpts<"PatchableFunctionEntryCount">>;
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def fms_hotpatch : Flag<["-"], "fms-hotpatch">, Group<f_Group>, Flags<[CC1Option, CoreOption]>,
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HelpText<"Ensure that all functions can be hotpatched at runtime">,
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MarshallingInfoFlag<CodeGenOpts<"HotPatch">>;
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def fpcc_struct_return : Flag<["-"], "fpcc-struct-return">, Group<f_Group>, Flags<[CC1Option]>,
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HelpText<"Override the default ABI to return all structs on the stack">;
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def fpch_preprocess : Flag<["-"], "fpch-preprocess">, Group<f_Group>;
@@ -6124,6 +6127,8 @@ def _SLASH_Gw_ : CLFlag<"Gw-">,
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def _SLASH_help : CLFlag<"help">, Alias<help>,
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HelpText<"Display available options">;
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def _SLASH_HELP : CLFlag<"HELP">, Alias<help>;
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def _SLASH_hotpatch : CLFlag<"hotpatch">, Alias<fms_hotpatch>,
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HelpText<"Create hotpatchable image">;
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def _SLASH_I : CLJoinedOrSeparate<"I">,
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HelpText<"Add directory to include search path">, MetaVarName<"<dir>">,
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Alias<I>;
@@ -6480,7 +6485,6 @@ def _SLASH_headerUnit : CLJoinedOrSeparate<"headerUnit">;
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def _SLASH_headerUnitAngle : CLJoinedOrSeparate<"headerUnit:angle">;
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def _SLASH_headerUnitQuote : CLJoinedOrSeparate<"headerUnit:quote">;
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def _SLASH_homeparams : CLFlag<"homeparams">;
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def _SLASH_hotpatch : CLFlag<"hotpatch">;
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def _SLASH_kernel : CLFlag<"kernel">;
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def _SLASH_LN : CLFlag<"LN">;
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def _SLASH_MP : CLJoined<"MP">;

‎clang/lib/CodeGen/BackendUtil.cpp

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@@ -645,6 +645,7 @@ static bool initTargetOptions(DiagnosticsEngine &Diags,
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Options.MCOptions.CommandLineArgs = CodeGenOpts.CommandLineArgs;
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Options.DebugStrictDwarf = CodeGenOpts.DebugStrictDwarf;
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Options.ObjectFilenameForDebug = CodeGenOpts.ObjectFilenameForDebug;
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Options.Hotpatch = CodeGenOpts.HotPatch;
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return true;
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}

‎clang/lib/CodeGen/CodeGenFunction.cpp

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@@ -882,6 +882,13 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
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if (Offset)
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Fn->addFnAttr("patchable-function-prefix", std::to_string(Offset));
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}
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// Instruct that functions for COFF/CodeView targets should start with a
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// patchable instruction, but only on x86/x64. Don't forward this to ARM/ARM64
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// backends as they don't need it -- instructions on these architectures are
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// always atomically patchable at runtime.
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if (CGM.getCodeGenOpts().HotPatch &&
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getContext().getTargetInfo().getTriple().isX86())
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Fn->addFnAttr("patchable-function", "prologue-short-redirect");
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// Add no-jump-tables value.
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if (CGM.getCodeGenOpts().NoUseJumpTables)

‎clang/lib/Driver/ToolChains/Clang.cpp

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@@ -6001,6 +6001,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
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}
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}
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Args.AddLastArg(CmdArgs, options::OPT_fms_hotpatch);
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if (TC.SupportsProfiling()) {
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Args.AddLastArg(CmdArgs, options::OPT_pg);
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‎clang/lib/Driver/ToolChains/MSVC.cpp

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@@ -511,6 +511,11 @@ void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
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if (Args.hasArg(options::OPT_g_Group, options::OPT__SLASH_Z7))
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CmdArgs.push_back("-debug");
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// If we specify /hotpatch, let the linker add padding in front of each
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// function, like MSVC does.
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if (Args.hasArg(options::OPT_fms_hotpatch, options::OPT__SLASH_hotpatch))
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CmdArgs.push_back("-functionpadmin");
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// Pass on /Brepro if it was passed to the compiler.
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// Note that /Brepro maps to -mno-incremental-linker-compatible.
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bool DefaultIncrementalLinkerCompatible =

‎clang/test/CodeGen/patchable-function-entry.c

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@@ -1,5 +1,6 @@
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// RUN: %clang_cc1 -triple aarch64 -emit-llvm %s -o - | FileCheck %s
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// RUN: %clang_cc1 -triple x86_64 -emit-llvm %s -fpatchable-function-entry=1 -o - | FileCheck --check-prefixes=CHECK,OPT %s
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// RUN: %clang_cc1 -triple x86_64 -emit-llvm %s -fms-hotpatch -o - | FileCheck --check-prefixes=HOTPATCH %s
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// CHECK: define{{.*}} void @f0() #0
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__attribute__((patchable_function_entry(0))) void f0() {}
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// CHECK: attributes #2 = { {{.*}} "patchable-function-entry"="0" "patchable-function-prefix"="4"
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// CHECK: attributes #3 = { {{.*}} "patchable-function-entry"="3" "patchable-function-prefix"="2"
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// OPT: attributes #4 = { {{.*}} "patchable-function-entry"="1"
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// HOTPATCH: attributes #0 = { {{.*}} "patchable-function"="prologue-short-redirect"
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// HOTPATCH: attributes #1 = { {{.*}} "patchable-function"="prologue-short-redirect"
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// HOTPATCH: attributes #2 = { {{.*}} "patchable-function"="prologue-short-redirect"
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// HOTPATCH: attributes #3 = { {{.*}} "patchable-function"="prologue-short-redirect"

‎clang/test/CodeGenCXX/debug-info-hotpatch-arm.cpp

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@@ -0,0 +1,26 @@
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// REQUIRES: aarch64-registered-target || arm-registered-target
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///
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/// Check that using /hotpatch doesn't generate an error.
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/// Binaries are always hotpatchable on ARM/ARM64.
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///
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// RUN: %clang_cl --target=arm-pc-windows-msvc /c /hotpatch /Z7 -- %s 2>&1
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// RUN: %clang_cl --target=aarch64-pc-windows-msvc /c /hotpatch /Z7 -- %s 2>&1
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///
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/// Ensure that we set the hotpatchable flag in the debug information.
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///
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// RUN: %clang_cl --target=arm-pc-windows-msvc /c /Z7 -o %t.obj -- %s
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// RUN: llvm-pdbutil dump -symbols %t.obj | FileCheck %s --check-prefix=HOTPATCH
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// RUN: %clang_cl --target=aarch64-pc-windows-msvc /c /Z7 -o %t.obj -- %s
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// RUN: llvm-pdbutil dump -symbols %t.obj | FileCheck %s --check-prefix=HOTPATCH
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// HOTPATCH: S_COMPILE3 [size = [[#]]]
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// HOTPATCH: flags = hot patchable
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///
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/// Unfortunately we need /Z7, Clang does not systematically generate S_COMPILE3.
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///
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// RUN: %clang_cl --target=aarch64-pc-windows-msvc /c -o %t.obj -- %s
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// RUN: llvm-pdbutil dump -symbols %t.obj | FileCheck %s --check-prefix=NO-HOTPATCH
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// NO-HOTPATCH-NOT: flags = hot patchable
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int main() {
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return 0;
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}

‎clang/test/CodeGenCXX/debug-info-hotpatch.cpp

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@@ -0,0 +1,20 @@
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// REQUIRES: x86-registered-target
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///
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// RUN: %clang_cl --target=x86_64-windows-msvc /c /hotpatch /Z7 -o %t.obj -- %s
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// RUN: llvm-pdbutil dump -symbols %t.obj | FileCheck %s --check-prefix=HOTPATCH
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// HOTPATCH: S_COMPILE3 [size = [[#]]]
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// HOTPATCH: flags = hot patchable
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///
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// RUN: %clang_cl --target=x86_64-windows-msvc /c /Z7 -o %t.obj -- %s
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// RUN: llvm-pdbutil dump -symbols %t.obj | FileCheck %s --check-prefix=NO-HOTPATCH
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// NO-HOTPATCH-NOT: flags = hot patchable
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///
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// RUN: %clang_cl --target=x86_64-windows-msvc /hotpatch -### -- %s 2>&1 \
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// RUN: | FileCheck %s --check-prefix=FUNCTIONPADMIN
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// FUNCTIONPADMIN: clang{{.*}}
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// FUNCTIONPADMIN: {{link.*"}}
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// FUNCTIONPADMIN: -functionpadmin
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int main() {
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return 0;
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}

‎clang/test/Driver/cl-options.c

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@@ -118,6 +118,9 @@
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// RUN: %clang_cl /Gw /Gw- -### -- %s 2>&1 | FileCheck -check-prefix=Gw_ %s
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// Gw_-NOT: -fdata-sections
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// RUN: %clang_cl /hotpatch -### -- %s 2>&1 | FileCheck -check-prefix=hotpatch %s
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// hotpatch: -fms-hotpatch
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// RUN: %clang_cl /Imyincludedir -### -- %s 2>&1 | FileCheck -check-prefix=SLASH_I %s
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// RUN: %clang_cl /I myincludedir -### -- %s 2>&1 | FileCheck -check-prefix=SLASH_I %s
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// SLASH_I: "-I" "myincludedir"
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// RUN: /GZ \
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// RUN: /H \
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// RUN: /homeparams \
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// RUN: /hotpatch \
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// RUN: /JMC \
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// RUN: /kernel \
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// RUN: /LN \

‎llvm/include/llvm/Target/TargetOptions.h

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@@ -140,9 +140,9 @@ namespace llvm {
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EnableMachineFunctionSplitter(false), SupportsDefaultOutlining(false),
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EmitAddrsig(false), EmitCallSiteInfo(false),
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SupportsDebugEntryValues(false), EnableDebugEntryValues(false),
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ValueTrackingVariableLocations(false),
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ForceDwarfFrameSection(false), XRayOmitFunctionIndex(false),
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DebugStrictDwarf(false),
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ValueTrackingVariableLocations(false), ForceDwarfFrameSection(false),
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XRayOmitFunctionIndex(false), DebugStrictDwarf(false),
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Hotpatch(false),
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FPDenormalMode(DenormalMode::IEEE, DenormalMode::IEEE) {}
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/// DisableFramePointerElim - This returns true if frame pointer elimination
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/// By default, it is set to false.
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unsigned DebugStrictDwarf : 1;
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345+
/// Emit the hotpatch flag in CodeView debug.
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unsigned Hotpatch : 1;
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/// Name of the stack usage file (i.e., .su file) if user passes
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/// -fstack-usage. If empty, it can be implied that -fstack-usage is not
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/// passed on the command line.

‎llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

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@@ -846,6 +846,12 @@ void CodeViewDebug::emitCompilerInformation() {
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if (MMI->getModule()->getProfileSummary(/*IsCS*/ false) != nullptr) {
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Flags |= static_cast<uint32_t>(CompileSym3Flags::PGO);
848848
}
849+
using ArchType = llvm::Triple::ArchType;
850+
ArchType Arch = Triple(MMI->getModule()->getTargetTriple()).getArch();
851+
if (Asm->TM.Options.Hotpatch || Arch == ArchType::thumb ||
852+
Arch == ArchType::aarch64) {
853+
Flags |= static_cast<uint32_t>(CompileSym3Flags::HotPatch);
854+
}
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850856
OS.AddComment("Flags and language");
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OS.emitInt32(Flags);

‎llvm/test/DebugInfo/COFF/ARMNT/arm-register-variables.ll

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@@ -24,7 +24,8 @@
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; OBJ: Compile3Sym {
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; OBJ-NEXT: Kind: S_COMPILE3 (0x113C)
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; OBJ-NEXT: Language: C (0x0)
27-
; OBJ-NEXT: Flags [ (0x0)
27+
; OBJ-NEXT: Flags [ (0x4000)
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; OBJ-NEXT: HotPatch (0x4000)
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; OBJ-NEXT: ]
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; OBJ-NEXT: Machine: ARMNT (0xF4)
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‎llvm/test/MC/AArch64/coff-debug.ll

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@@ -77,7 +77,8 @@ attributes #0 = { noinline nounwind optnone "correctly-rounded-divide-sqrt-fp-ma
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; CHECK: Compile3Sym {
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; CHECK: Kind: S_COMPILE3 (0x113C)
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; CHECK: Language: C (0x0)
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; CHECK: Flags [ (0x0)
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; CHECK: Flags [ (0x4000
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; CHECK: HotPatch (0x4000)
8182
; CHECK: ]
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; CHECK: }
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; CHECK: ]

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