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

[Nick Desaulniers via libc-commits]

https://github.com/nickdesaulniers created https://github.com/llvm/llvm-project/pull/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%).


>From b96823a869a42ae2978363ce56f3a1ad42c7b1ec Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers at google.com>
Date: Fri, 19 Jan 2024 11:05:34 -0800
Subject: [PATCH] [libc] default enable -ftrivial-auto-var-init=pattern

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%).
---
 libc/cmake/modules/LLVMLibCObjectRules.cmake               | 5 +++++
 utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl | 7 ++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

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