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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

<details>
<summary>Changes</summary>

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%).


---
Full diff: https://github.com/llvm/llvm-project/pull/78776.diff


2 Files Affected:

- (modified) libc/cmake/modules/LLVMLibCObjectRules.cmake (+5) 
- (modified) utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl (+6-1) 


``````````diff
diff --git a/libc/cmake/modules/LLVMLibCObjectRules.cmake b/libc/cmake/modules/LLVMLibCObjectRules.cmake
index 6eba17ae9201c8..46f5ed5cd7465f 100644
--- a/libc/cmake/modules/LLVMLibCObjectRules.cmake
+++ b/libc/cmake/modules/LLVMLibCObjectRules.cmake
@@ -41,6 +41,11 @@ 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")
+    # clang-8+, gcc-12+
+    check_cxx_compiler_flag("-ftrivial-auto-var-init=pattern" LIBC_CC_SUPPORTS_PATTERN_INIT)
+    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 cf27001be9dfed..a28fa51a39f99d 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

``````````

</details>


https://github.com/llvm/llvm-project/pull/78776