[Lldb-commits] [lldb] r342029 - Reduce alignment on struct XSAVE, fixing a gcc warning

Pavel Labath via lldb-commits lldb-commits at lists.llvm.org
Wed Sep 12 01:50:08 PDT 2018

Author: labath
Date: Wed Sep 12 01:50:08 2018
New Revision: 342029

URL: http://llvm.org/viewvc/llvm-project?rev=342029&view=rev
Reduce alignment on struct XSAVE, fixing a gcc warning

The warning is about heap-allocating a struct with bigger alignment
requirements than the standard heap allocator provides.

AFAICT, all uses of the XSAVE struct are already heap-allocated, so this
high alignment does not actually have any effect and removing it should
be NFC.

I have also done some digging in the commit history. This alignment
requirement was since the XSAVE struct was introduced in r180572 when
adding AVX register support for linux. It does not mention the alignment
specifically, so I am guessing this was just put there because the
corresponging XSAVE cpu instruction requires its buffer to be 64-byte
aligned. However, LLDB will not be normally reading this struct via the
XSAVE instruction directly. Instead we will ask the kernel to copy the
buffer saved when suspeding the inferior. This should not require such
strict alignment (in fact, linux kernel will happily do this for any


Modified: lldb/trunk/source/Plugins/Process/Utility/RegisterContext_x86.h
URL: http://llvm.org/viewvc/llvm-project/lldb/trunk/source/Plugins/Process/Utility/RegisterContext_x86.h?rev=342029&r1=342028&r2=342029&view=diff
--- lldb/trunk/source/Plugins/Process/Utility/RegisterContext_x86.h (original)
+++ lldb/trunk/source/Plugins/Process/Utility/RegisterContext_x86.h Wed Sep 12 01:50:08 2018
@@ -341,7 +341,7 @@ LLVM_PACKED_END
 // x86 extensions to FXSAVE (i.e. for AVX and MPX processors)
-struct LLVM_ALIGNAS(64) XSAVE {
+struct LLVM_ALIGNAS(16) XSAVE {
   FXSAVE i387;      // floating point registers typical in i387_fxsave_struct
   XSAVE_HDR header; // The xsave_hdr_struct can be used to determine if the
                     // following extensions are usable

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