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

Mark Kettenis
2010-03-07T20:29:03Z

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> Date: Sun, 7 Mar 2010 08:40:10 -0800
> From: "H.J. Lu" <hjl.tools@gmail.com>
>
> 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?

A possible solution here is to simply always map %xmmN onto %ymmN if
the target supports AVX. This'll make "info address" say that a
128-bit vector variable lives in %ymmN instead of %xmmN, but that
wouldn't really be a lie, would it? The only problem with this
approach is that it will break cases where the debug info refers to a
variable living in consecutive %xmm (128-bit) registers using (only)
the register number of the first %xmm register. This shouldn't happen
with DWARF2, but might happen with older debug formats like stabs.
Not necessarily a serious probem; at least nothing I care about still
uses stabs.

Should be a simple matter of returning the %ymm pseudo register number
if tdep->num_ymm_regs > 0.

> My first approach works here since XMM and YMM register have the same
> register numbers. We can solve it with 2 alternatives:

Works or does not work?

> 1. Give a different DWARF register number for YMM register,
> which is an incompatible ABI change.

That really is the only viable option if you want to dismbiguate the
%ymm registers from the %xmm registers.

> 2. Implement YMM registers as a super set of XMM registers, which
> is my first approach.

I don't think this really solves anything. You'll still have issues
with values stored in consecutive registers.