[PATCH] D18479: [CodeGenCXX] Fix ItaniumCXXABI::getAlignmentOfExnObject to return 8-byte alignment on Darwin

[Akira Hatanaka via cfe-commits]

> On Mar 25, 2016, at 2:06 PM, David Majnemer via cfe-commits <cfe-commits at lists.llvm.org> wrote:
> 
> 
> 
> On Fri, Mar 25, 2016 at 12:57 PM, Akira Hatanaka via cfe-commits <cfe-commits at lists.llvm.org <mailto:cfe-commits at lists.llvm.org>> wrote:
> ahatanak created this revision.
> ahatanak added a reviewer: rjmccall.
> ahatanak added a subscriber: cfe-commits.
> 
> r246985 made changes to give a higher alignment for exception objects on the grounds that Itanium says _Unwind_Exception should be "double-word" aligned and the structure is normally declared with __attribute__((aligned)) guaranteeing 16-byte alignment. It turns out that libc++abi doesn't declare the structure with __attribute__((aligned)) and therefore only guarantees 8-byte alignment on 32-bit and 64-bit platforms. This caused a crash in some cases when the backend emitted SIMD store instructions that requires 16-byte alignment (such as movaps).
> 
> This patch makes ItaniumCXXABI::getAlignmentOfExnObject return an 8-byte alignment on Darwin to fix the crash.
> 
> http://reviews.llvm.org/D18479 <http://reviews.llvm.org/D18479>
> 
> Files:
>   lib/CodeGen/ItaniumCXXABI.cpp
>   test/CodeGenCXX/eh.cpp
> 
> Index: test/CodeGenCXX/eh.cpp
> ===================================================================
> --- test/CodeGenCXX/eh.cpp
> +++ test/CodeGenCXX/eh.cpp
> @@ -448,5 +448,27 @@
>    }
>  }
> 
> +namespace test17 {
> +class BaseException {
> +private:
> +  int a[4];
> +public:
> +  BaseException() {};
> +};
> +
> +class DerivedException: public BaseException {
> +};
> +
> +int foo() {
> +  throw DerivedException();
> +  // The alignment passed to memset is 8, not 16, on Darwin.
> +
> +  // CHECK: [[T0:%.*]] = call i8* @__cxa_allocate_exception(i64 16)
> +  // CHECK-NEXT: [[T1:%.*]] = bitcast i8* [[T0]] to %"class.test17::DerivedException"*
> +  // CHECK-NEXT: [[T2:%.*]] = bitcast %"class.test17::DerivedException"* [[T1]] to i8*
> +  // CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* [[T2]], i8 0, i64 16, i32 8, i1 false)
> +}
> +}
> +
>  // CHECK: attributes [[NUW]] = { nounwind }
>  // CHECK: attributes [[NR]] = { noreturn }
> Index: lib/CodeGen/ItaniumCXXABI.cpp
> ===================================================================
> --- lib/CodeGen/ItaniumCXXABI.cpp
> +++ lib/CodeGen/ItaniumCXXABI.cpp
> @@ -163,8 +163,17 @@
>    /// we assume that alignment here.  (It's generally 16 bytes, but
>    /// some targets overwrite it.)
>    CharUnits getAlignmentOfExnObject() {
> -    auto align = CGM.getContext().getTargetDefaultAlignForAttributeAligned();
> -    return CGM.getContext().toCharUnitsFromBits(align);
> +    unsigned Align;
> +
> +    // Alignment is 8 for darwin since libc++abi doesn't declare
> +    // _Unwind_Exception with __attribute__((aligned)) and therefore doesn't
> +    // guarantee 16-byte alignment.
> +    if (CGM.getContext().getTargetInfo().getTriple().isOSDarwin())
> 
> What about Linux/FreeBSD targets which use libc++abi?
>  

Is there a way to detect whether libc++abi is used?

> +      Align = 64;
> +    else
> +      Align = CGM.getContext().getTargetDefaultAlignForAttributeAligned();
> +
> +    return CGM.getContext().toCharUnitsFromBits(Align);
>    }
> 
>    void emitRethrow(CodeGenFunction &CGF, bool isNoReturn) override;
> 
> 
> 
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