MachineFunction: -fsanitize={function,kcfi}: ensure 4-byte alignment

Fix https://github.com/llvm/llvm-project/issues/63579
```
% cat a.c
void foo() {}
% clang --target=arm-none-eabi -mthumb -mno-unaligned-access -fsanitize=kcfi a.c -S -o - | grep p2align
        .p2align        1
% clang --target=armv6m-none-eabi -fsanitize=function a.c -S -o - | grep p2align
        .p2align        1
```

Ensure that -fsanitize={function,kcfi} instrumented functions are aligned by at
least 4, so that loading the type hash before the function label will not cause
a misaligned access. This is especially important for -mno-unaligned-access
configurations that don't set `setMinFunctionAlignment` to 4 or greater.

With this patch, the generated assembly for the examples above will contain `.p2align 2`
before the type hash.

If `__attribute__((aligned(N)))` or `-falign-functions=N` is specified, the
larger alignment will be used.

Reviewed By: simon_tatham, samitolvanen

Differential Revision: https://reviews.llvm.org/D154125

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineFunction.cpp b/llvm/lib/CodeGen/MachineFunction.cpp
index a934a5c..88939e9 100644
--- a/llvm/lib/CodeGen/MachineFunction.cpp
+++ b/llvm/lib/CodeGen/MachineFunction.cpp
@@ -212,6 +212,14 @@ void MachineFunction::init() {
     Alignment = std::max(Alignment,
                          STI->getTargetLowering()->getPrefFunctionAlignment());
 
+  // -fsanitize=function and -fsanitize=kcfi instrument indirect function calls
+  // to load a type hash before the function label. Ensure functions are aligned
+  // by a least 4 to avoid unaligned access, which is especially important for
+  // -mno-unaligned-access.
+  if (F.hasMetadata(LLVMContext::MD_func_sanitize) ||
+      F.getMetadata(LLVMContext::MD_kcfi_type))
+    Alignment = std::max(Alignment, Align(4));
+
   if (AlignAllFunctions)
     Alignment = Align(1ULL << AlignAllFunctions);