Extract libc variant implementation commit for upstreaming preparation #1

Copilot · 2025-10-13T11:09:27Z

This PR extracts commit 56310430f940e5b45de5b337a5b00609097f48e1 from the upstream llvm-project repository into a separate branch to prepare it for upstreaming.

Changes

This commit adds a generic variant implementation for POD types to the LLVM libc project, providing a type-safe alternative to raw unions for use in libc components.

New Files Added

libc/src/__support/CPP/variant.h (256 lines)
- Standalone implementation of std::variant for POD types
- Supports 2 and 3 type specializations
- Assumes trivially copyable and destructible types for simplified implementation
- Provides get<T>() methods for type-safe access
- Includes both mutable and const access methods
libc/test/src/__support/CPP/variant_test.cpp (119 lines)
- Comprehensive test suite covering:
  - Default construction
  - Construction from specific types
  - Copy construction and assignment
  - Type-safe access via get<T>()
  - Move semantics
  - Const correctness
  - Three-type variant support

Modified Files

libc/src/__support/CPP/CMakeLists.txt
- Added variant header library with appropriate dependencies on type_traits, utility, and macro headers
libc/test/src/__support/CPP/CMakeLists.txt
- Added variant_test to the libc-cpp-utils-tests suite

Verification

All file contents have been verified to match the original upstream commit exactly via blob hash comparison. The commit is ready for upstreaming to the main LLVM project.

Original commit: llvm@5631043
Author: Jeff Bailey [email protected]
Date: Fri Oct 3 22:42:07 2025 +0000

Original prompt

Please take llvm@5631043 and separate it into its own branch so that I can prepare it for upstreaming.

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- Add variant.h supporting 2 and 3 type specializations for POD types - Assumes trivially copyable/destructible types for simplified implementation - Add comprehensive variant_test.cpp with tests for construction, assignment, type-safe access, and move semantics - Place variant and variant_test in correct alphabetical order in CMakeLists.txt - Remove variant test from time_zone_posix_test.cpp (moved to proper location) This provides a type-safe alternative to raw unions for use in libc components.

Otherwise debug-info is stripped, which influences the language of the current frame. Also, set explicit breakpoint because Windows seems to not obey the debugtrap. Log from failing test on Windows: ``` (lldb) command source -s 0 'lit-lldb-init-quiet' Executing commands in 'D:\test\lit-lldb-init-quiet'. (lldb) command source -C --silent-run true lit-lldb-init (lldb) target create "main.out" Current executable set to 'D:\test\main.out' (x86_64). (lldb) settings set interpreter.stop-command-source-on-error false (lldb) command source -s 0 'with-target.input' Executing commands in 'D:\test\with-target.input'. (lldb) expr blah ^ error: use of undeclared identifier 'blah' note: Falling back to default language. Ran expression as 'Objective C++'. (lldb) run Process 29404 launched: 'D:\test\main.out' (x86_64) Process 29404 stopped * thread #1, stop reason = Exception 0x80000003 encountered at address 0x7ff7b3df7189 frame #0: 0x00007ff7b3df718a main.out -> 0x7ff7b3df718a: xorl %eax, %eax 0x7ff7b3df718c: popq %rcx 0x7ff7b3df718d: retq 0x7ff7b3df718e: int3 (lldb) expr blah ^ error: use of undeclared identifier 'blah' note: Falling back to default language. Ran expression as 'Objective C++'. (lldb) expr -l objc -- blah ^ error: use of undeclared identifier 'blah' note: Expression evaluation in pure Objective-C not supported. Ran expression as 'Objective C++'. (lldb) expr -l c -- blah ^ error: use of undeclared identifier 'blah' note: Expression evaluation in pure C not supported. Ran expression as 'ISO C++'. ```

The Tkinter module was renamed to tkinter in Python 3.0. https://docs.python.org/2/library/tkinter.html https://docs.python.org/3/library/tkinter.html Rest of it appears to work when imported inside of LLDB: ``` $ ./bin/lldb /tmp/test.o (lldb) target create "/tmp/test.o" Current executable set to '/tmp/test.o' (x86_64). (lldb) b main Breakpoint 1: where = test.o`main + 8 at test.c:1:18, address = 0x0000000000001131 (lldb) run Process 121572 launched: '/tmp/test.o' (x86_64) Process 121572 stopped * thread #1, name = 'test.o', stop reason = breakpoint 1.1 frame #0: 0x0000555555555131 test.o`main at test.c:1:18 -> 1 int main() { int a = 1; char b = '?'; return 0; } (lldb) command script import <...>/llvm-project/lldb/examples/python/lldbtk.py (lldb) tk- Available completions: tk-process -- For more information run 'help tk-process' tk-target -- For more information run 'help tk-target' tk-variables -- For more information run 'help tk-variables' (lldb) tk-process (lldb) tk-target (lldb) tk-variables ```

…ypes (llvm#162278) When we take the following C program: ``` int main() { return 0; } ``` and create a statically-linked executable from it: ``` clang -static -g -o main main.c ``` Then we can observe the following `lldb` behavior: ``` $ lldb (lldb) target create main Current executable set to '.../main' (x86_64). (lldb) breakpoint set --name main Breakpoint 1: where = main`main + 11 at main.c:2:3, address = 0x000000000022aa7b (lldb) process launch Process 3773637 launched: '/home/me/tmp/built-in/main' (x86_64) Process 3773637 stopped * thread #1, name = 'main', stop reason = breakpoint 1.1 frame #0: 0x000000000022aa7b main`main at main.c:2:3 1 int main() { -> 2 return 0; 3 } (lldb) script lldb.debugger.GetSelectedTarget().FindFirstType("__int128").size 0 (lldb) script lldb.debugger.GetSelectedTarget().FindFirstType("unsigned __int128").size 0 (lldb) quit ``` The value return by the `SBTarget::FindFirstType` method is wrong for the `__int128` and `unsigned __int128` basic types. The proposed changes make the `TypeSystemClang::GetBasicTypeEnumeration` method consistent with `gcc` and `clang` C [language extension](https://gcc.gnu.org/onlinedocs/gcc/_005f_005fint128.html) related to 128-bit integer types as well as with the `BuiltinType::getName` method in the LLVM codebase itself. When the above change is applied, the behavior of the `lldb` changes in the following (desired) way: ``` $ lldb (lldb) target create main Current executable set to '.../main' (x86_64). (lldb) breakpoint set --name main Breakpoint 1: where = main`main + 11 at main.c:2:3, address = 0x000000000022aa7b (lldb) process launch Process 3773637 launched: '/home/me/tmp/built-in/main' (x86_64) Process 3773637 stopped * thread #1, name = 'main', stop reason = breakpoint 1.1 frame #0: 0x000000000022aa7b main`main at main.c:2:3 1 int main() { -> 2 return 0; 3 } (lldb) script lldb.debugger.GetSelectedTarget().FindFirstType("__int128").size 16 (lldb) script lldb.debugger.GetSelectedTarget().FindFirstType("unsigned __int128").size 16 (lldb) quit ``` --------- Co-authored-by: Matej Košík <[email protected]>

**Mitigation for:** google/sanitizers#749 **Disclosure:** I'm not an ASan compiler expert yet (I'm trying to learn!), I primarily work in the runtime. Some of this PR was developed with the help of AI tools (primarily as a "fuzzy `grep` engine"), but I've manually refined and tested the output, and can speak for every line. In general, I used it only to orient myself and for "rubberducking". **Context:** The msvc ASan team (👋 ) has received an internal request to improve clang's exception handling under ASan for Windows. Namely, we're interested in **mitigating** this bug: google/sanitizers#749 To summarize, today, clang + ASan produces a false-positive error for this program: ```C++ #include <cstdio> #include <exception> int main() { try { throw std::exception("test"); }catch (const std::exception& ex){ puts(ex.what()); } return 0; } ``` The error reads as such: ``` C:\Users\dajusto\source\repros\upstream>type main.cpp #include <cstdio> #include <exception> int main() { try { throw std::exception("test"); }catch (const std::exception& ex){ puts(ex.what()); } return 0; } C:\Users\dajusto\source\repros\upstream>"C:\Users\dajusto\source\repos\llvm-project\build.runtimes\bin\clang.exe" -fsanitize=address -g -O0 main.cpp C:\Users\dajusto\source\repros\upstream>a.exe ================================================================= ==19112==ERROR: AddressSanitizer: access-violation on unknown address 0x000000000000 (pc 0x7ff72c7c11d9 bp 0x0080000ff960 sp 0x0080000fcf50 T0) ==19112==The signal is caused by a READ memory access. ==19112==Hint: address points to the zero page. #0 0x7ff72c7c11d8 in main C:\Users\dajusto\source\repros\upstream\main.cpp:8 #1 0x7ff72c7d479f in _CallSettingFrame C:\repos\msvc\src\vctools\crt\vcruntime\src\eh\amd64\handlers.asm:49 llvm#2 0x7ff72c7c8944 in __FrameHandler3::CxxCallCatchBlock(struct _EXCEPTION_RECORD *) C:\repos\msvc\src\vctools\crt\vcruntime\src\eh\frame.cpp:1567 llvm#3 0x7ffb4a90e3e5 (C:\WINDOWS\SYSTEM32\ntdll.dll+0x18012e3e5) llvm#4 0x7ff72c7c1128 in main C:\Users\dajusto\source\repros\upstream\main.cpp:6 llvm#5 0x7ff72c7c33db in invoke_main C:\repos\msvc\src\vctools\crt\vcstartup\src\startup\exe_common.inl:78 llvm#6 0x7ff72c7c33db in __scrt_common_main_seh C:\repos\msvc\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288 llvm#7 0x7ffb49b05c06 (C:\WINDOWS\System32\KERNEL32.DLL+0x180035c06) llvm#8 0x7ffb4a8455ef (C:\WINDOWS\SYSTEM32\ntdll.dll+0x1800655ef) ==19112==Register values: rax = 0 rbx = 80000ff8e0 rcx = 27d76d00000 rdx = 80000ff8e0 rdi = 80000fdd50 rsi = 80000ff6a0 rbp = 80000ff960 rsp = 80000fcf50 r8 = 100 r9 = 19930520 r10 = 8000503a90 r11 = 80000fd540 r12 = 80000fd020 r13 = 0 r14 = 80000fdeb8 r15 = 0 AddressSanitizer can not provide additional info. SUMMARY: AddressSanitizer: access-violation C:\Users\dajusto\source\repros\upstream\main.cpp:8 in main ==19112==ABORTING ``` The root of the issue _appears to be_ that ASan's instrumentation is incompatible with Window's assumptions for instantiating `catch`-block's parameters (`ex` in the snippet above). The nitty gritty details are lost on me, but I understand that to make this work without loss of ASan coverage, a "serious" refactoring is needed. In the meantime, users risk false positive errors when pairing ASan + catch-block parameters on Windows. **To mitigate this** I think we should avoid instrumenting catch-block parameters on Windows. It appears to me this is as "simple" as marking catch block parameters as "uninteresting" in `AddressSanitizer::isInterestingAlloca`. My manual tests seem to confirm this. I believe this is strictly better than today's status quo, where the runtime generates false positives. Although we're now explicitly choosing to instrument less, the benefit is that now more programs can run with ASan without _funky_ macros that disable ASan on exception blocks. **This PR:** implements the mitigation above, and creates a simple new test for it. _Thanks!_ --------- Co-authored-by: Antonio Frighetto <[email protected]>

Initial plan

70de19d

Copilot AI assigned Copilot and kaladron Oct 13, 2025

Copilot started work on behalf of kaladron October 13, 2025 11:10 View session

Copilot AI changed the title ~~[WIP] Separate commit for upstreaming~~ Extract libc variant implementation commit for upstreaming preparation Oct 13, 2025

Copilot AI requested a review from kaladron October 13, 2025 11:27

Copilot finished work on behalf of kaladron October 13, 2025 11:27

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Extract libc variant implementation commit for upstreaming preparation #1

Extract libc variant implementation commit for upstreaming preparation #1

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