[BOLT] Evaluate and speedup discoverFileObjects

[aaupov]

Operations with symbols such as name lookup are expensive and some binaries have millions of symbols, so it makes sense to try to reduce the symbol-related overheads.

Measure how much the following cost and refactor/eliminate:

1. Checks for symbol name being equal to __asan_init and __llvm_coverage_mapping for every single symbol:

   llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

   Lines 824 to 837 in a1423ba

   	for (const ELFSymbolRef &Symbol : InputFile->symbols()) {
   	Expected<StringRef> NameOrError = Symbol.getName();
   	if (NameOrError && NameOrError->starts_with("__asan_init")) {
   	BC->errs()
   	<< "BOLT-ERROR: input file was compiled or linked with sanitizer "
   	"support. Cannot optimize.\n";
   	exit(1);
   	}
   	if (NameOrError && NameOrError->starts_with("__llvm_coverage_mapping")) {
   	BC->errs()
   	<< "BOLT-ERROR: input file was compiled or linked with coverage "
   	"support. Cannot optimize.\n";
   	exit(1);
   	}

2. FileName set for every local function symbol (with O(1M) for large binaries) while we can instead use file symbol lookup with O(log N_Syms) cost and FileSyms memory cost [BOLT][NFCI] Use FileSymbols for local symbol disambiguation #89088
3. Merge iteration over two loops over symbols into one:

   llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

   Line 824 in a1423ba

   for (const ELFSymbolRef &Symbol : InputFile->symbols()) {

   and

   llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

   Line 876 in a1423ba

   for (const SymbolRef &Symbol : InputFile->symbols())

[llvmbot]

@llvm/issue-subscribers-bolt

Author: Amir Ayupov (aaupov)

Operations with symbols such as name lookup are expensive and some binaries have millions of symbols, so it makes sense to try to reduce the symbol-related overheads.

Measure how much the following cost and refactor/eliminate:

1. Checks for symbol name being equal to __asan_init and __llvm_coverage_mapping for every single symbol:

   llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

   Lines 824 to 837 in a1423ba

   	for (const ELFSymbolRef &Symbol : InputFile->symbols()) {
   	Expected<StringRef> NameOrError = Symbol.getName();
   	if (NameOrError && NameOrError->starts_with("__asan_init")) {
   	BC->errs()
   	<< "BOLT-ERROR: input file was compiled or linked with sanitizer "
   	"support. Cannot optimize.\n";
   	exit(1);
   	}
   	if (NameOrError && NameOrError->starts_with("__llvm_coverage_mapping")) {
   	BC->errs()
   	<< "BOLT-ERROR: input file was compiled or linked with coverage "
   	"support. Cannot optimize.\n";
   	exit(1);
   	}

2. FileName set for every local function symbol (with O(1M) for large binaries) while we can instead use file symbol lookup with O(log N_Syms) cost and FileSyms memory cost [BOLT][NFCI] Use FileSymbols for local symbol disambiguation #89088
3. Merge iteration over two loops over symbols into one:

   llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

   Line 824 in a1423ba

   for (const ELFSymbolRef &Symbol : InputFile->symbols()) {

   and

   llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

   Line 876 in a1423ba

   for (const SymbolRef &Symbol : InputFile->symbols())

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Amir Ayupov (aaupov)

Operations with symbols such as name lookup are expensive and some binaries have millions of symbols, so it makes sense to try to reduce the symbol-related overheads.

Measure how much the following cost and refactor/eliminate:

1. Checks for symbol name being equal to __asan_init and __llvm_coverage_mapping for every single symbol:

   llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

   Lines 824 to 837 in a1423ba

   	for (const ELFSymbolRef &Symbol : InputFile->symbols()) {
   	Expected<StringRef> NameOrError = Symbol.getName();
   	if (NameOrError && NameOrError->starts_with("__asan_init")) {
   	BC->errs()
   	<< "BOLT-ERROR: input file was compiled or linked with sanitizer "
   	"support. Cannot optimize.\n";
   	exit(1);
   	}
   	if (NameOrError && NameOrError->starts_with("__llvm_coverage_mapping")) {
   	BC->errs()
   	<< "BOLT-ERROR: input file was compiled or linked with coverage "
   	"support. Cannot optimize.\n";
   	exit(1);
   	}

2. FileName set for every local function symbol (with O(1M) for large binaries) while we can instead use file symbol lookup with O(log N_Syms) cost and FileSyms memory cost [BOLT][NFCI] Use FileSymbols for local symbol disambiguation #89088
3. Merge iteration over two loops over symbols into one:

   llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

   Line 824 in a1423ba

   for (const ELFSymbolRef &Symbol : InputFile->symbols()) {

   and

   llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

   Line 876 in a1423ba

   for (const SymbolRef &Symbol : InputFile->symbols())

[corona10]

Well, Can I take a look at it?

[aaupov]

Well, Can I take a look at it?

Yes, go for it!

[corona10]

@aaupov
Do you have any recommended projects as the target binary to measure the performance overhead?

[aaupov]

@aaupov Do you have any recommended projects as the target binary to measure the performance overhead?

The largest that you can get your hands on, the more symbols the better. Or CPython if it's the one that you care about the most. Clang is a reasonably large, important, and straightforward project to work with.

[corona10]

There are several issues (zero stack count.. :( ) when converting to the flame graph,
but I success in getting the information about discoverFileObjects. It is not huge stuff for the CPython build (But it's worth checking it from Chromium or Clang)
(For the CPython, around 10513 symbols were evaluated)

Anyway, I will start to optimize with following what you proposed :)

[corona10]

@aaupov
By the way, I noticed that there is a code that sorts 1M symbols through the rewriting process.

llvm-project/bolt/lib/Rewrite/RewriteInstance.cpp

Line 899 in a1423ba

llvm::stable_sort(SortedSymbols, CompareSymbols);

Even if the vector is an efficient data structure for most cases, it will require huge time complexity.
Do you think that it's worth optimizing to use a sorted data structure?
(O(n) + O(nlogn) -> O(nlogn), will be kind of constant time optimization.)

[aaupov]

I don't think there's much opportunity here. Let's keep this task open to address points 1 and 2.

[corona10]

So, I have questions about point 1.

Checks for symbol name being equal to __asan_init and __llvm_coverage_mapping for every single symbol:

So, do you want to make NameOrError == anas_init_string or compare the string with all __asan_init variations?
These are pretty naive questions, but due to my lack of background knowledge, I don't know why the original author used starts with operation for this, but to solve this issue, I should know.

When I searched about __asan_init there are kind of variation exists such as __asan_init_v2 or __asan_init_v3