Re: PATCH: 0/6 [2nd try]: Add AVX support

Mark Kettenis
2010-03-07T20:00:32Z

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> Date: Sun, 7 Mar 2010 09:39:44 -0800
> From: "H.J. Lu" <hjl.tools@gmail.com>
>
> On Sun, Mar 7, 2010 at 9:04 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> > On Sun, Mar 7, 2010 at 8:40 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> >> On Sun, Mar 7, 2010 at 8:31 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> >>> On Sun, Mar 7, 2010 at 6:37 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> >>>> On Sun, Mar 7, 2010 at 6:16 AM, Mark Kettenis <mark.kettenis@xs4all.nl> wrote:
> >>>>>> Date: Sat, 6 Mar 2010 14:16:34 -0800
> >>>>>> From: "H.J. Lu" <hongjiu.lu@intel.com>
> >>>>>>
> >>>>>> AVX registers are saved and restored via the XSAVE extended state. The
> >>>>>> extended control register 0 (the XFEATURE_ENABLED_MASK register), XCR0,
> >>>>>> is used to determine which states, x87, SSE, AVX, ... are supported
> >>>>>> in the XSAVE extended state.  XCR0 can be read with the new "xgetbv"
> >>>>>> instruction.  The xstate_bv field at byte offset 512 in the XSAVE
> >>>>>> extended state indicates what states the current process is in. If
> >>>>>> the feature bit is cleared, the corresponding registers should be read as
> >>>>>> 0. If we update a register, we should set the corresponding feature
> >>>>>> bit in the xstate_bv field.
> >>>>>>
> >>>>>> We added PTRACE_GETREGSET and PTRACE_SETREGSET to Linux kernel to
> >>>>>> fetch and store AVX registers with ptrace. Linux kernel also stores
> >>>>>> XCR0 at the first 8 bytes of the software usable bytes, starting at
> >>>>>> byte offset 464.
> >>>>>>
> >>>>>> There are total 6 patches to add AVX support for Linux.  They support:
> >>>>>>
> >>>>>> 1. The upper 128bit YMM registers are added for AVX support. The upper
> >>>>>> 128bit YMM registers are hidden from users. Gdb combines XMM register,
> >>>>>> %xmmX, with 128bit YMM register, %ymmXh, and present the whole 256bit
> >>>>>> YMM register, %ymmX, as pseudo register to users.
> >>>>>> 2. Backward compatible. If AVX isn't supported, SSE will be used.
> >>>>>> 3. Forward compatible. If new state beyond AVX is supported in
> >>>>>> the XSAVE extended state, only AVX state will be used.
> >>>>>> 4. Remote gdb protocol extension. GDB will send "x86=xml" in qSupported
> >>>>>> request packet to indicate that GDB supports x86 XML target desciption.
> >>>>>> The gdb stub will send x86 XML target desciption if it sees "x86=xml"
> >>>>>> in qSupported request packet.
> >>>>>>
> >>>>>> One advantage of this approach is YMM registers are actually stored as
> >>>>>> XMM registers and upper YMM registers in the XSAVE extended state.  It
> >>>>>> is easy and natural to access them as %xmmX and %ymmXh internally.  We
> >>>>>> just need to hide %ymmXh from users.
> >>>>>>
> >>>>>> To support AVX on other OSes, the following changes are needed:
> >>>>>>
> >>>>>> 1. Kernel support to get/set the XSAVE extended state.
> >>>>>> 2. Handle 8/16 upper YMM registers.
> >>>>>> 3. Provide target to_read_description to return SSE or AVX target
> >>>>>> description.
> >>>>>> 4. Update gdbarch_core_read_description to return SSE or AVX target
> >>>>>> description based on contents of core dump.
> >>>>>
> >>>>> Wait; there is something important missing here.  How are the new %ymm
> >>>>> registers referred to in debug info?  The AMD64 SysV psABI defines the
> >>>>> DWARF register Number Mapping, but the 0.99.4 draft copy I have
> >>>>> doesn't define any mappings for the %ymm registers.  What mapping does
> >>>>> GCC use?
> >>>>>
> >>>>
> >>>> In gcc, XMM and YMM registers have the same register number. They map
> >>>> to be the same DWARF register with different sizes.  Since XMM and YMM
> >>>> registers are caller-saved, they don't appear in unwind info. So, the same
> >>>> DWARF register with different sizes for XMM/YMM registers isn't a problem.
> >>>>
> >>>>
> >>>
> >>> Yes, there is a problem. amd64_dwarf_reg_to_regnum needs to map 256bit
> >>> register to YMM. How do other arches solve this?
> >>>
> >>
> >> My first approach works here since XMM and YMM register have the same
> >> register numbers.  We can solve it with 2 alternatives:
> >>
> >> 1. Give a different DWARF register number for YMM register,
> >> which is an incompatible ABI change.
> >> 2. Implement YMM registers as a super set of XMM registers, which
> >> is my first approach.
> >>
> >> Thanks Mark for pointing out this issue.
> >
> > Or I can provide i386_value_from_register.

Probably a bad idea. That function really is only intended to convert
values that have a completely different bit-representation in the
register.

> It doesn't work on x86 since i386_value_from_register will change
> regum. Can we change
>
> typedef struct value * (gdbarch_value_from_register_ftype) (struct
> type *type, int regnum, struct frame_info *frame);
>
> to
>
> typedef struct value * (gdbarch_value_from_register_ftype) (struct
> type *type, int *regnum, struct frame_info *frame);
>
> to support updating regnum?

I'd rather not.