phosek updated this revision to Diff 230733.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D69740/new/
https://reviews.llvm.org/D69740
Files:
clang/docs/SourceBasedCodeCoverage.rst
clang/lib/Driver/ToolChains/Darwin.cpp
compiler-rt/lib/profile/CMakeLists.txt
compiler-rt/lib/profile/InstrProfiling.h
compiler-rt/lib/profile/InstrProfilingBiasVar.c
compiler-rt/lib/profile/InstrProfilingBuffer.c
compiler-rt/lib/profile/InstrProfilingFile.c
compiler-rt/lib/profile/InstrProfilingInternal.h
compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c
llvm/include/llvm/ProfileData/InstrProf.h
llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
llvm/test/Instrumentation/InstrProfiling/runtime-counter-relocation.ll
Index: llvm/test/Instrumentation/InstrProfiling/runtime-counter-relocation.ll
===================================================================
--- /dev/null
+++ llvm/test/Instrumentation/InstrProfiling/runtime-counter-relocation.ll
@@ -0,0 +1,25 @@
+; RUN: opt < %s -S -instrprof | FileCheck %s
+; RUN: opt < %s -S -instrprof -runtime-counter-relocation | FileCheck -check-prefixes=RELOC %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+@__profn_foo = hidden constant [3 x i8] c"foo"
+; RELOC: @__llvm_profile_counter_bias = linkonce_odr global i64 0
+
+; CHECK-LABEL: define void @foo
+; CHECK-NEXT: %pgocount = load i64, i64* getelementptr inbounds ([1 x i64], [1 x i64]* @__profc_foo, i64 0, i64 0)
+; CHECK-NEXT: %1 = add i64 %pgocount, 1
+; CHECK-NEXT: store i64 %1, i64* getelementptr inbounds ([1 x i64], [1 x i64]* @__profc_foo, i64 0, i64 0)
+; RELOC-LABEL: define void @foo
+; RELOC-NEXT: %1 = load i64, i64* @__llvm_profile_counter_bias
+; RELOC-NEXT: %2 = add i64 ptrtoint ([1 x i64]* @__profc_foo to i64), %1
+; RELOC-NEXT: %3 = inttoptr i64 %2 to i64*
+; RELOC-NEXT: %pgocount = load i64, i64* %3
+; RELOC-NEXT: %4 = add i64 %pgocount, 1
+; RELOC-NEXT: store i64 %4, i64* %3
+define void @foo() {
+ call void @llvm.instrprof.increment(i8* getelementptr inbounds ([3 x i8], [3 x i8]* @__profn_foo, i32 0, i32 0), i64 0, i32 1, i32 0)
+ ret void
+}
+
+declare void @llvm.instrprof.increment(i8*, i64, i32, i32)
Index: llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
===================================================================
--- llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
+++ llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
@@ -83,6 +83,11 @@
cl::desc("Rename counter variable of a comdat function based on cfg hash"),
cl::init(true));
+cl::opt<bool> RuntimeCounterRelocation(
+ "runtime-counter-relocation",
+ cl::desc("Enable relocating counters at runtime."),
+ cl::init(false));
+
cl::opt<bool> ValueProfileStaticAlloc(
"vp-static-alloc",
cl::desc("Do static counter allocation for value profiler"),
@@ -408,6 +413,20 @@
}
bool InstrProfiling::lowerIntrinsics(Function *F) {
+ Value *Load = nullptr;
+ if (isRuntimeCounterRelocationEnabled()) {
+ if (!F->empty()) {
+ IRBuilder<> Builder(&F->getEntryBlock().front());
+ Type *Int64Ty = Type::getInt64Ty(M->getContext());
+ GlobalVariable *Bias = M->getGlobalVariable(getInstrProfCounterBiasVarName());
+ if (!Bias)
+ Bias = new GlobalVariable(*M, Int64Ty, false, GlobalValue::LinkOnceODRLinkage,
+ Constant::getNullValue(Int64Ty),
+ getInstrProfCounterBiasVarName());
+ Load = Builder.CreateLoad(Int64Ty, Bias);
+ }
+ }
+
bool MadeChange = false;
PromotionCandidates.clear();
for (BasicBlock &BB : *F) {
@@ -424,13 +443,23 @@
}
}
- if (!MadeChange)
+ if (!MadeChange) {
+ if (RuntimeCounterRelocation && Load)
+ Load->deleteValue();
return false;
+ }
promoteCounterLoadStores(F);
return true;
}
+bool InstrProfiling::isRuntimeCounterRelocationEnabled() const {
+ if (RuntimeCounterRelocation.getNumOccurrences() > 0)
+ return RuntimeCounterRelocation;
+
+ return TT.isOSFuchsia();
+}
+
bool InstrProfiling::isCounterPromotionEnabled() const {
if (DoCounterPromotion.getNumOccurrences() > 0)
return DoCounterPromotion;
@@ -641,6 +670,17 @@
Value *Addr = Builder.CreateConstInBoundsGEP2_64(Counters->getValueType(),
Counters, 0, Index);
+ if (isRuntimeCounterRelocationEnabled()) {
+ Type *Int64Ty = Type::getInt64Ty(M->getContext());
+ Type *Int64PtrTy = Type::getInt64PtrTy(M->getContext());
+ Function *Fn = Inc->getParent()->getParent();
+ Instruction &I = Fn->getEntryBlock().front();
+ if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
+ auto *Add = Builder.CreateAdd(Builder.CreatePtrToInt(Addr, Int64Ty), LI);
+ Addr = Builder.CreateIntToPtr(Add, Int64PtrTy);
+ }
+ }
+
if (Options.Atomic || AtomicCounterUpdateAll) {
Builder.CreateAtomicRMW(AtomicRMWInst::Add, Addr, Inc->getStep(),
AtomicOrdering::Monotonic);
Index: llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
===================================================================
--- llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
+++ llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
@@ -82,6 +82,9 @@
/// Register-promote counter loads and stores in loops.
void promoteCounterLoadStores(Function *F);
+ /// Returns true if relocating counters at runtime is enabled.
+ bool isRuntimeCounterRelocationEnabled() const;
+
/// Returns true if profile counter update register promotion is enabled.
bool isCounterPromotionEnabled() const;
Index: llvm/include/llvm/ProfileData/InstrProf.h
===================================================================
--- llvm/include/llvm/ProfileData/InstrProf.h
+++ llvm/include/llvm/ProfileData/InstrProf.h
@@ -153,6 +153,10 @@
return "__llvm_profile_runtime_user";
}
+inline StringRef getInstrProfCounterBiasVarName() {
+ return "__llvm_profile_counter_bias";
+}
+
/// Return the marker used to separate PGO names during serialization.
inline StringRef getInstrProfNameSeparator() { return "\01"; }
Index: compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c
===================================================================
--- compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c
+++ compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c
@@ -34,16 +34,10 @@
#include "InstrProfilingInternal.h"
#include "InstrProfilingUtil.h"
-/* VMO that contains the coverage data shared across all modules. This symbol
- * has default visibility and is exported in each module (executable or DSO)
- * that statically links in the profiling runtime.
- */
-zx_handle_t __llvm_profile_vmo;
-/* Current offset within the VMO where data should be written next. This symbol
- * has default visibility and is exported in each module (executable or DSO)
- * that statically links in the profiling runtime.
- */
-uint64_t __llvm_profile_offset;
+/* VMO that contains the coverage data shared across all modules. */
+static zx_handle_t __llvm_profile_vmo;
+/* Current offset within the VMO where data should be written next. */
+static uint64_t __llvm_profile_offset;
static const char ProfileSinkName[] = "llvm-profile";
@@ -58,55 +52,6 @@
__sanitizer_log_write(s, ret + 1);
}
-static void createVMO() {
- /* Don't create VMO if it has been alread created. */
- if (__llvm_profile_vmo != ZX_HANDLE_INVALID)
- return;
-
- /* Get information about the current process. */
- zx_info_handle_basic_t Info;
- zx_status_t Status =
- _zx_object_get_info(_zx_process_self(), ZX_INFO_HANDLE_BASIC, &Info,
- sizeof(Info), NULL, NULL);
- if (Status != ZX_OK) {
- lprofWrite("LLVM Profile: cannot get info about current process: %s\n",
- _zx_status_get_string(Status));
- return;
- }
-
- /* Create VMO to hold the profile data. */
- Status = _zx_vmo_create(0, ZX_VMO_RESIZABLE, &__llvm_profile_vmo);
- if (Status != ZX_OK) {
- lprofWrite("LLVM Profile: cannot create VMO: %s\n",
- _zx_status_get_string(Status));
- return;
- }
-
- /* Give the VMO a name including our process KOID so it's easy to spot. */
- char VmoName[ZX_MAX_NAME_LEN];
- snprintf(VmoName, sizeof(VmoName), "%s.%" PRIu64, ProfileSinkName, Info.koid);
- _zx_object_set_property(__llvm_profile_vmo, ZX_PROP_NAME, VmoName,
- strlen(VmoName));
-
- /* Duplicate the handle since __sanitizer_publish_data consumes it. */
- zx_handle_t Handle;
- Status =
- _zx_handle_duplicate(__llvm_profile_vmo, ZX_RIGHT_SAME_RIGHTS, &Handle);
- if (Status != ZX_OK) {
- lprofWrite("LLVM Profile: cannot duplicate VMO handle: %s\n",
- _zx_status_get_string(Status));
- _zx_handle_close(__llvm_profile_vmo);
- __llvm_profile_vmo = ZX_HANDLE_INVALID;
- return;
- }
-
- /* Publish the VMO which contains profile data to the system. */
- __sanitizer_publish_data(ProfileSinkName, Handle);
-
- /* Use the dumpfile symbolizer markup element to write the name of VMO. */
- lprofWrite("LLVM Profile: {{{dumpfile:%s:%s}}}\n", ProfileSinkName, VmoName);
-}
-
static uint32_t lprofVMOWriter(ProfDataWriter *This, ProfDataIOVec *IOVecs,
uint32_t NumIOVecs) {
/* Compute the total length of data to be written. */
@@ -173,6 +118,101 @@
static void dumpWithoutReturn(void) { dump(); }
+static void createVMO(void) {
+ /* Don't create VMO if it has been alread created. */
+ if (__llvm_profile_vmo != ZX_HANDLE_INVALID)
+ return;
+
+ const __llvm_profile_data *DataBegin = __llvm_profile_begin_data();
+ const __llvm_profile_data *DataEnd = __llvm_profile_end_data();
+ const uint64_t DataSize = __llvm_profile_get_data_size(DataBegin, DataEnd);
+ const uint64_t CountersOffset = sizeof(__llvm_profile_header) +
+ (DataSize * sizeof(__llvm_profile_data));
+
+ /* Get information about the current process. */
+ zx_info_handle_basic_t Info;
+ zx_status_t Status =
+ _zx_object_get_info(_zx_process_self(), ZX_INFO_HANDLE_BASIC, &Info,
+ sizeof(Info), NULL, NULL);
+ if (Status != ZX_OK) {
+ lprofWrite("LLVM Profile: cannot get info about current process: %s\n",
+ _zx_status_get_string(Status));
+ return;
+ }
+
+ /* Create VMO to hold the profile data. */
+ Status = _zx_vmo_create(0, ZX_VMO_RESIZABLE, &__llvm_profile_vmo);
+ if (Status != ZX_OK) {
+ lprofWrite("LLVM Profile: cannot create VMO: %s\n",
+ _zx_status_get_string(Status));
+ return;
+ }
+
+ /* Give the VMO a name including our process KOID so it's easy to spot. */
+ char VmoName[ZX_MAX_NAME_LEN];
+ snprintf(VmoName, sizeof(VmoName), "%s.%" PRIu64, ProfileSinkName, //Info.koid,
+ lprofGetLoadModuleSignature());
+ _zx_object_set_property(__llvm_profile_vmo, ZX_PROP_NAME, VmoName,
+ strlen(VmoName));
+
+ /* Duplicate the handle since __sanitizer_publish_data consumes it. */
+ zx_handle_t Handle;
+ Status =
+ _zx_handle_duplicate(__llvm_profile_vmo, ZX_RIGHT_SAME_RIGHTS, &Handle);
+ if (Status != ZX_OK) {
+ lprofWrite("LLVM Profile: cannot duplicate VMO handle: %s\n",
+ _zx_status_get_string(Status));
+ _zx_handle_close(__llvm_profile_vmo);
+ __llvm_profile_vmo = ZX_HANDLE_INVALID;
+ return;
+ }
+
+ /* Publish the VMO which contains profile data to the system. */
+ __sanitizer_publish_data(ProfileSinkName, Handle);
+
+ /* Use the dumpfile symbolizer markup element to write the name of VMO. */
+ lprofWrite("LLVM Profile: {{{dumpfile:%s:%s}}}\n", ProfileSinkName, VmoName);
+
+ /* Check if there is llvm/runtime version mismatch. */
+ if (GET_VERSION(__llvm_profile_get_version()) != INSTR_PROF_RAW_VERSION) {
+ lprofWrite("LLVM Profile: runtime and instrumentation version mismatch: "
+ "expected %d, but got %d\n",
+ INSTR_PROF_RAW_VERSION,
+ (int)GET_VERSION(__llvm_profile_get_version()));
+ return;
+ }
+
+ /* Write the profile data into the mapped region. */
+ ProfDataWriter VMOWriter;
+ initVMOWriter(&VMOWriter);
+ if (lprofWriteData(&VMOWriter, 0, 0) != 0) {
+ lprofWrite("LLVM Profile: failed to write data\n");
+ return;
+ }
+
+ uint64_t Len = 0;
+ Status = _zx_vmo_get_size(__llvm_profile_vmo, &Len);
+ if (Status != ZX_OK) {
+ lprofWrite("LLVM Profile: failed to get the VMO size: %s\n",
+ _zx_status_get_string(Status));
+ return;
+ }
+
+ uintptr_t Mapping;
+ Status =
+ _zx_vmar_map(_zx_vmar_root_self(), ZX_VM_PERM_READ | ZX_VM_PERM_WRITE,
+ 0, __llvm_profile_vmo, 0, Len, &Mapping);
+ if (Status != ZX_OK) {
+ lprofWrite("LLVM Profile: failed to map the VMO: %s\n",
+ _zx_status_get_string(Status));
+ return;
+ }
+
+ /* Update the profile fields based on the current mapping. */
+ __llvm_profile_counter_bias = (intptr_t)Mapping -
+ (uintptr_t)__llvm_profile_begin_counters() + CountersOffset;
+}
+
/* This method is invoked by the runtime initialization hook
* InstrProfilingRuntime.o if it is linked in.
*/
Index: compiler-rt/lib/profile/InstrProfilingInternal.h
===================================================================
--- compiler-rt/lib/profile/InstrProfilingInternal.h
+++ compiler-rt/lib/profile/InstrProfilingInternal.h
@@ -177,6 +177,7 @@
unsigned lprofProfileDumped();
void lprofSetProfileDumped();
+COMPILER_RT_VISIBILITY extern int RuntimeCounterRelocation;
COMPILER_RT_VISIBILITY extern void (*FreeHook)(void *);
COMPILER_RT_VISIBILITY extern uint8_t *DynamicBufferIOBuffer;
COMPILER_RT_VISIBILITY extern uint32_t VPBufferSize;
Index: compiler-rt/lib/profile/InstrProfilingFile.c
===================================================================
--- compiler-rt/lib/profile/InstrProfilingFile.c
+++ compiler-rt/lib/profile/InstrProfilingFile.c
@@ -436,6 +436,98 @@
}
#endif // !defined(__Fuchsia__) && !defined(_WIN32)
+static int writeMMappedFile(FILE *OutputFile, char **Profile) {
+ if (!OutputFile)
+ return -1;
+
+ /* Write the data into a file. */
+ setupIOBuffer();
+ ProfDataWriter fileWriter;
+ initFileWriter(&fileWriter, OutputFile);
+ if (lprofWriteData(&fileWriter, NULL, 0)) {
+ PROF_ERR("Failed to write profile: %s\n", strerror(errno));
+ return -1;
+ }
+ fflush(OutputFile);
+
+ /* Get the file size. */
+ uint64_t FileSize = ftell(OutputFile);
+
+ /* Map the profile. */
+ *Profile = (char *)mmap(
+ NULL, FileSize, PROT_READ | PROT_WRITE, MAP_SHARED, fileno(OutputFile), 0);
+ if (*Profile == MAP_FAILED) {
+ PROF_ERR("Unable to mmap profile: %s\n", strerror(errno));
+ return -1;
+ }
+
+ return 0;
+}
+
+static void relocateCounters(void) {
+ if (!__llvm_profile_is_continuous_mode_enabled() || !RuntimeCounterRelocation)
+ return;
+
+ /* Get the sizes of various profile data sections. Taken from
+ * __llvm_profile_get_size_for_buffer(). */
+ const __llvm_profile_data *DataBegin = __llvm_profile_begin_data();
+ const __llvm_profile_data *DataEnd = __llvm_profile_end_data();
+ uint64_t DataSize = __llvm_profile_get_data_size(DataBegin, DataEnd);
+ const uint64_t CountersOffset = sizeof(__llvm_profile_header) +
+ (DataSize * sizeof(__llvm_profile_data));
+
+ int Length = getCurFilenameLength();
+ char *FilenameBuf = (char *)COMPILER_RT_ALLOCA(Length + 1);
+ const char *Filename = getCurFilename(FilenameBuf, 0);
+ if (!Filename)
+ return;
+
+ FILE *File = NULL;
+ char *Profile = NULL;
+
+ if (!doMerging()) {
+ File = fopen(Filename, "w+b");
+ if (!File)
+ return;
+
+ if (writeMMappedFile(File, &Profile) == -1) {
+ fclose(File);
+ return;
+ }
+ } else {
+ File = lprofOpenFileEx(Filename);
+ if (!File)
+ return;
+
+ uint64_t ProfileFileSize = 0;
+ if (getProfileFileSizeForMerging(File, &ProfileFileSize) == -1) {
+ lprofUnlockFileHandle(File);
+ fclose(File);
+ return;
+ }
+
+ if (!ProfileFileSize) {
+ if (writeMMappedFile(File, &Profile) == -1) {
+ fclose(File);
+ return;
+ }
+ } else {
+ /* The merged profile has a non-zero length. Check that it is compatible
+ * with the data in this process. */
+ if (mmapProfileForMerging(File, ProfileFileSize, &Profile) == -1) {
+ fclose(File);
+ return;
+ }
+ }
+
+ lprofUnlockFileHandle(File);
+ }
+
+ /* Update the profile fields based on the current mapping. */
+ __llvm_profile_counter_bias = (intptr_t)Profile -
+ (uintptr_t)__llvm_profile_begin_counters() + CountersOffset;
+}
+
static void initializeProfileForContinuousMode(void) {
if (!__llvm_profile_is_continuous_mode_enabled())
return;
@@ -695,7 +787,12 @@
}
truncateCurrentFile();
- initializeProfileForContinuousMode();
+ if (__llvm_profile_is_continuous_mode_enabled()) {
+ if (RuntimeCounterRelocation)
+ relocateCounters();
+ else
+ initializeProfileForContinuousMode();
+ }
}
/* Return buffer length that is required to store the current profile
@@ -845,6 +942,9 @@
ProfileNameSpecifier PNS = PNS_unknown;
int hasCommandLineOverrider = (INSTR_PROF_PROFILE_NAME_VAR[0] != 0);
+ if (__llvm_profile_counter_bias != -1)
+ RuntimeCounterRelocation = 1;
+
EnvFilenamePat = getFilenamePatFromEnv();
if (EnvFilenamePat) {
/* Pass CopyFilenamePat = 1, to ensure that the filename would be valid
Index: compiler-rt/lib/profile/InstrProfilingBuffer.c
===================================================================
--- compiler-rt/lib/profile/InstrProfilingBuffer.c
+++ compiler-rt/lib/profile/InstrProfilingBuffer.c
@@ -10,6 +10,9 @@
#include "InstrProfilingInternal.h"
#include "InstrProfilingPort.h"
+/* When counters are being relocated at runtime, this parameter is set to 1. */
+COMPILER_RT_VISIBILITY int RuntimeCounterRelocation = 0;
+
/* When continuous mode is enabled (%c), this parameter is set to 1.
*
* This parameter is defined here in InstrProfilingBuffer.o, instead of in
@@ -62,7 +65,8 @@
uint64_t DataSize, uint64_t CountersSize, uint64_t NamesSize,
uint64_t *PaddingBytesBeforeCounters, uint64_t *PaddingBytesAfterCounters,
uint64_t *PaddingBytesAfterNames) {
- if (!__llvm_profile_is_continuous_mode_enabled()) {
+ if (!__llvm_profile_is_continuous_mode_enabled() &&
+ !RuntimeCounterRelocation) {
*PaddingBytesBeforeCounters = 0;
*PaddingBytesAfterCounters = 0;
*PaddingBytesAfterNames = __llvm_profile_get_num_padding_bytes(NamesSize);
Index: compiler-rt/lib/profile/InstrProfilingBiasVar.c
===================================================================
--- /dev/null
+++ compiler-rt/lib/profile/InstrProfilingBiasVar.c
@@ -0,0 +1,15 @@
+/*===- InstrProfilingBiasVar.c - profile counter bias variable setup ------===*\
+|*
+|* 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
+|*
+\*===----------------------------------------------------------------------===*/
+
+#include "InstrProfiling.h"
+
+/* The runtime should only provide its own definition of this symbol when the
+ * user has not specified one. Set this up by moving the runtime's copy of this
+ * symbol to an object file within the archive.
+ */
+COMPILER_RT_WEAK intptr_t __llvm_profile_counter_bias = -1;
Index: compiler-rt/lib/profile/InstrProfiling.h
===================================================================
--- compiler-rt/lib/profile/InstrProfiling.h
+++ compiler-rt/lib/profile/InstrProfiling.h
@@ -307,4 +307,11 @@
*/
extern char INSTR_PROF_PROFILE_NAME_VAR[1]; /* __llvm_profile_filename. */
+/*!
+ * This variable is a weak symbol defined in InstrProfilingBiasVar.c. It
+ * allows compiler instrumentation to provide overriding definition with
+ * value from compiler command line. This variable has hidden visibility.
+ */
+COMPILER_RT_VISIBILITY extern intptr_t __llvm_profile_counter_bias;
+
#endif /* PROFILE_INSTRPROFILING_H_ */
Index: compiler-rt/lib/profile/CMakeLists.txt
===================================================================
--- compiler-rt/lib/profile/CMakeLists.txt
+++ compiler-rt/lib/profile/CMakeLists.txt
@@ -52,6 +52,7 @@
GCDAProfiling.c
InstrProfiling.c
InstrProfilingValue.c
+ InstrProfilingBiasVar.c
InstrProfilingBuffer.c
InstrProfilingFile.c
InstrProfilingMerge.c
Index: clang/lib/Driver/ToolChains/Darwin.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Darwin.cpp
+++ clang/lib/Driver/ToolChains/Darwin.cpp
@@ -1147,6 +1147,7 @@
} else {
addExportedSymbol(CmdArgs, "___llvm_profile_filename");
addExportedSymbol(CmdArgs, "___llvm_profile_raw_version");
+ addExportedSymbol(CmdArgs, "___llvm_profile_counter_bias");
}
addExportedSymbol(CmdArgs, "_lprofDirMode");
}
Index: clang/docs/SourceBasedCodeCoverage.rst
===================================================================
--- clang/docs/SourceBasedCodeCoverage.rst
+++ clang/docs/SourceBasedCodeCoverage.rst
@@ -92,15 +92,42 @@
instrumented program crashes, or is killed by a signal, perfect coverage
information can still be recovered. Continuous mode does not support value
profiling for PGO, and is only supported on Darwin at the moment. Support for
- Linux may be mostly complete but requires testing, and support for
- Fuchsia/Windows may require more extensive changes: please get involved if
- you are interested in porting this feature.
+ Linux may be mostly complete but requires testing, and support for Windows
+ may require more extensive changes: please get involved if you are interested
+ in porting this feature.
.. code-block:: console
# Step 2: Run the program.
% LLVM_PROFILE_FILE="foo.profraw" ./foo
+Note that continuous mode is also used on Fuchsia where it's the only supported
+mode, but the implementation is different. The Darwin and Linux implementation
+relies on padding and the ability to map a file over the existing memory
+mapping which is generally only available on POSIX systems and isn't suitable
+for other platforms.
+
+On Fuchsia, we rely on the the ability to relocate counters at runtime using a
+level of indirection. On every counter access, we add a bias to the counter
+address. This bias is stored in ``__llvm_profile_counter_bias`` symbol that's
+provided by the profile runtime and is initially set to zero, meaning no
+relocation. The runtime can map the profile into memory at abitrary location,
+and set bias to the offset between the original and the new counter location,
+at which point every subsequent counter access will be to the new location,
+which allows updating profile directly akin to the continous mode.
+
+The advantage of this approach is that doesn't require any special OS support.
+The disadvantage is the extra overhead due to additional instructions required
+for each counter access (overhead both in terms of binary size and performance)
+plus duplication of counters (i.e. one copy in the binary itself and another
+copy that's mapped into memory). This implementation can be also enabled for
+other platforms by passing the ``-runtime-counter-relocation`` option to the
+backend during compilation.
+
+.. code-block:: console
+
+ % clang++ -fprofile-instr-generate -fcoverage-mapping -mllvm -runtime-counter-relocation foo.cc -o foo
+
Creating coverage reports
=========================
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