[libc-commits] [libc] 1d5c16d - [libc] default enable -ftrivial-auto-var-init=pattern (#78776)

[via libc-commits]

Author: Nick Desaulniers
Date: 2024-01-22T14:55:51-08:00
New Revision: 1d5c16d7805806e920feee2b4ec93d5800293837

URL: https://github.com/llvm/llvm-project/commit/1d5c16d7805806e920feee2b4ec93d5800293837
DIFF: https://github.com/llvm/llvm-project/commit/1d5c16d7805806e920feee2b4ec93d5800293837.diff

LOG: [libc] default enable -ftrivial-auto-var-init=pattern (#78776)

Usage of uninitialized memory is a top memory safety issue in C++ codebases.
Help mitigate this somewhat by default initialize stack allocations to a
pattern (0xAA repeating).

Clang has received optimizations to sink these into control flow paths that
access such values to minimize the overhead of these added initializations.

If there's a measurable slowdown, we can add
-ftrivial-auto-var-init-max-size=<N> for some value N bytes if we have any
large stack allocations, or add attribute uninitialized to any variable
declarations.

Unsupported until GCC 12.1 / Clang 8.

Increases file size of libc.a from a full build by +8.79Ki (+0.2%).

Added: 
    

Modified: 
    libc/cmake/modules/CheckCompilerFeatures.cmake
    libc/cmake/modules/LLVMLibCObjectRules.cmake
    utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl

Removed: 
    


################################################################################
diff  --git a/libc/cmake/modules/CheckCompilerFeatures.cmake b/libc/cmake/modules/CheckCompilerFeatures.cmake
index 1ad81bbb5f666ce..c99a98d1567d325 100644
--- a/libc/cmake/modules/CheckCompilerFeatures.cmake
+++ b/libc/cmake/modules/CheckCompilerFeatures.cmake
@@ -57,3 +57,8 @@ foreach(feature IN LISTS ALL_COMPILER_FEATURES)
 endforeach()
 
 message(STATUS "Compiler features available: ${AVAILABLE_COMPILER_FEATURES}")
+
+### Compiler Feature Detection ###
+
+# clang-8+, gcc-12+
+check_cxx_compiler_flag("-ftrivial-auto-var-init=pattern" LIBC_CC_SUPPORTS_PATTERN_INIT)

diff  --git a/libc/cmake/modules/LLVMLibCObjectRules.cmake b/libc/cmake/modules/LLVMLibCObjectRules.cmake
index 6eba17ae9201c83..b6c2f2d53e88d29 100644
--- a/libc/cmake/modules/LLVMLibCObjectRules.cmake
+++ b/libc/cmake/modules/LLVMLibCObjectRules.cmake
@@ -41,6 +41,9 @@ function(_get_common_compile_options output_var flags)
     list(APPEND compile_options "-fno-unwind-tables")
     list(APPEND compile_options "-fno-asynchronous-unwind-tables")
     list(APPEND compile_options "-fno-rtti")
+    if (LIBC_CC_SUPPORTS_PATTERN_INIT)
+      list(APPEND compile_options "-ftrivial-auto-var-init=pattern")
+    endif()
     list(APPEND compile_options "-Wall")
     list(APPEND compile_options "-Wextra")
     # -DLIBC_WNO_ERROR=ON if you can't build cleanly with -Werror.

diff  --git a/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl b/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl
index cf27001be9dfed6..a28fa51a39f99de 100644
--- a/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl
+++ b/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl
@@ -81,7 +81,12 @@ def libc_function(
     # We use the explicit equals pattern here because append and += mutate the
     # original list, where this creates a new list and stores it in deps.
     copts = copts or []
-    copts = copts + ["-O3", "-fno-builtin", "-fno-lax-vector-conversions"]
+    copts = copts + [
+      "-O3",
+      "-fno-builtin",
+      "-fno-lax-vector-conversions",
+      "-ftrivial-auto-var-init=pattern"
+    ]
 
     # We compile the code twice, the first target is suffixed with ".__internal__" and contains the
     # C++ functions in the "LIBC_NAMESPACE" namespace. This allows us to test the function in the