[profile] Add a mode to continuously sync counter updates to a file

Add support for continuously syncing profile counter updates to a file.

The motivation for this is that programs do not always exit cleanly. On
iOS, for example, programs are usually killed via a signal from the OS.
Running atexit() handlers after catching a signal is unreliable, so some
method for progressively writing out profile data is necessary.

The approach taken here is to mmap() the `__llvm_prf_cnts` section onto
a raw profile. To do this, the linker must page-align the counter and
data sections, and the runtime must ensure that counters are mapped to a
page-aligned offset within a raw profile.

Continuous mode is (for the moment) incompatible with the online merging
mode. This limitation is lifted in https://reviews.llvm.org/D69586.

Continuous mode is also (for the moment) incompatible with value
profiling, as I'm not sure whether there is interest in this and the
implementation may be tricky.

As I have not been able to test extensively on non-Darwin platforms,
only Darwin support is included for the moment. However, continuous mode
may "just work" without modification on Linux and some UNIX-likes. AIUI
the default value for the GNU linker's `--section-alignment` flag is set
to the page size on many systems. This appears to be true for LLD as
well, as its `no_nmagic` option is on by default. Continuous mode will
not "just work" on Fuchsia or Windows, as it's not possible to mmap() a
section on these platforms. There is a proposal to add a layer of
indirection to the profile instrumentation to support these platforms.

rdar://54210980

Differential Revision: https://reviews.llvm.org/D68351

1 files changed, 35 insertions, 1 deletions

diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
index e15f089b0d7..d8c18effd62 100644
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -19,6 +19,7 @@
 #include "clang/Driver/SanitizerArgs.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Option/ArgList.h"
+#include "llvm/ProfileData/InstrProf.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include "llvm/Support/TargetParser.h"
@@ -1110,6 +1111,19 @@ static void addExportedSymbol(ArgStringList &CmdArgs, const char *Symbol) {
   CmdArgs.push_back(Symbol);
 }
 
+/// Add a sectalign directive for \p Segment and \p Section to the maximum
+/// expected page size for Darwin.
+///
+/// On iPhone 6+ the max supported page size is 16K. On macOS, the max is 4K.
+/// Use a common alignment constant (16K) for now, and reduce the alignment on
+/// macOS if it proves important.
+static void addSectalignToPage(const ArgList &Args, ArgStringList &CmdArgs,
+                               StringRef Segment, StringRef Section) {
+  for (const char *A : {"-sectalign", Args.MakeArgString(Segment),
+                        Args.MakeArgString(Section), "0x4000"})
+    CmdArgs.push_back(A);
+}
+
 void Darwin::addProfileRTLibs(const ArgList &Args,
                               ArgStringList &CmdArgs) const {
   if (!needsProfileRT(Args)) return;
@@ -1117,11 +1131,13 @@ void Darwin::addProfileRTLibs(const ArgList &Args,
   AddLinkRuntimeLib(Args, CmdArgs, "profile",
                     RuntimeLinkOptions(RLO_AlwaysLink | RLO_FirstLink));
 
+  bool ForGCOV = needsGCovInstrumentation(Args);
+
   // If we have a symbol export directive and we're linking in the profile
   // runtime, automatically export symbols necessary to implement some of the
   // runtime's functionality.
   if (hasExportSymbolDirective(Args)) {
-    if (needsGCovInstrumentation(Args)) {
+    if (ForGCOV) {
       addExportedSymbol(CmdArgs, "___gcov_flush");
       addExportedSymbol(CmdArgs, "_flush_fn_list");
       addExportedSymbol(CmdArgs, "_writeout_fn_list");
@@ -1131,6 +1147,24 @@ void Darwin::addProfileRTLibs(const ArgList &Args,
     }
     addExportedSymbol(CmdArgs, "_lprofDirMode");
   }
+
+  // Align __llvm_prf_{cnts,data} sections to the maximum expected page
+  // alignment. This allows profile counters to be mmap()'d to disk. Note that
+  // it's not enough to just page-align __llvm_prf_cnts: the following section
+  // must also be page-aligned so that its data is not clobbered by mmap().
+  //
+  // The section alignment is only needed when continuous profile sync is
+  // enabled, but this is expected to be the default in Xcode. Specifying the
+  // extra alignment also allows the same binary to be used with/without sync
+  // enabled.
+  if (!ForGCOV) {
+    for (auto IPSK : {llvm::IPSK_cnts, llvm::IPSK_data}) {
+      addSectalignToPage(
+          Args, CmdArgs, "__DATA",
+          llvm::getInstrProfSectionName(IPSK, llvm::Triple::MachO,
+                                        /*AddSegmentInfo=*/false));
+    }
+  }
 }
 
 void DarwinClang::AddLinkSanitizerLibArgs(const ArgList &Args,