llvm dev - Apr 2021 - [RFC] [X86] Emit unaligned vector moves on avx machine with option control.

| This sounds like the -fmax-type-align flag:

Well, no, at least not for the PS4 case.  In our case, the type had an alignment
attribute but the caller didn’t make sure the allocated memory was aligned
properly.  The -fmax-type-align flag explicitly doesn’t do anything in that
case, if I’m reading it correctly.  (Yes, it’s a bug.  Yes, sanitizers or other
testing could have found it.  No, there is no opportunity to do any of the
things that would have fixed it correctly.)

Really what we did was effectively this:  Pretend X86 doesn’t have a VMOVAPS
opcode.  That’s all.  Nothing about memory/operand alignment attributes was
modified, IR is unchanged.  Pretend that one machine opcode is missing.  Can’t
possibly affect anything about IR optimizations, *maybe* something post-ISel
would be different but even that is hard to imagine.  (As best I can remember,
the only test updates we had to make were to change things like “vmovaps” to
“vmov{{u|a}}ps” and done.)  It’s like we did s/movaps/movups/g on the assembly
output.

I still can’t say I think it should be appropriate to do upstream—no real info
yet on Intel’s problem case--but I hope this explains why the bigger hammer
(i.e., get Clang involved) doesn’t seem necessary or appropriate.
--paulr

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Reid
Kleckner via llvm-dev
Sent: Thursday, April 15, 2021 12:59 PM
To: James Y Knight <jyknight at google.com>
Cc: llvm-dev at lists.llvm.org; Liu, Chen3 <chen3.liu at intel.com>; Luo,
Yuanke <yuanke.luo at intel.com>; Maslov, Sergey V <sergey.v.maslov at
intel.com>
Subject: Re: [llvm-dev] [RFC] [X86] Emit unaligned vector moves on avx machine
with option control.

On Wed, Apr 14, 2021 at 11:58 AM James Y Knight via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
What I suspect you actually want here is an option to tell Clang not to infer
load/store alignments based on object types or alignment attributes -- instead
treating everything as being potentially aligned to 1 unless the allocation is
seen (e.g. global/local variables). Clang would still need to use the usual
alignment computation for variable definitions and structure layout, but not
memory operations. If clang emits "load ... align 1" instructions in
LLVM IR, the right thing would then happen in the X86 backend automatically.

This sounds like the -fmax-type-align flag:
https://clang.llvm.org/docs/UsersManual.html#controlling-code-generation<https://urldefense.com/v3/__https:/clang.llvm.org/docs/UsersManual.html*controlling-code-generation__;Iw!!JmoZiZGBv3RvKRSx!uoBVF33nyuM5lbseJ-XKanIeYhdhHW9yOoxyF7zJ56FjUs8jsfdUcuw4AQ96FRBrmA$>
Explicit alignment attributes are still honored, so some aligned vector
instructions may be generated. However, the documentation describes essentially
this exact use case.
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Right, I get that this doesn't match what you are doing for PS4, and it
doesn't match what Chen3 Liu proposed. To James's point, the
-fmax-type-align flag is more principled because it powers down all the
other LLVM optimizations that assume aligned pointers have zeros in the low
bits.

As for how to handle explicit alignment attributes that don't come from
type information, maybe we could revisit that behavior, or conditionalize
it with a flag. I just mean to say that there is prior art for this
direction. We should continue in the direction of a complete solution from
the frontend, rather than adding a workaround in the backend.

On Thu, Apr 15, 2021 at 11:54 AM <paul.robinson at sony.com> wrote:
> | This sounds like the -fmax-type-align flag:
>
>
>
> Well, no, at least not for the PS4 case.  In our case, the type had an
> alignment attribute but the caller didn’t make sure the allocated memory
> was aligned properly.  The -fmax-type-align flag explicitly doesn’t do
> anything in that case, if I’m reading it correctly.  (Yes, it’s a bug.
> Yes, sanitizers or other testing could have found it.  No, there is no
> opportunity to do any of the things that would have fixed it correctly.)
>
>
>
> Really what we did was effectively this:  Pretend X86 doesn’t have a
> VMOVAPS opcode.  That’s all.  Nothing about memory/operand alignment
> attributes was modified, IR is unchanged.  Pretend that one machine opcode
> is missing.  Can’t possibly affect anything about IR optimizations, *
> *maybe** something post-ISel would be different but even that is hard to
> imagine.  (As best I can remember, the only test updates we had to make
> were to change things like “vmovaps” to “vmov{{u|a}}ps” and done.)  It’s
> like we did s/movaps/movups/g on the assembly output.
>
>
>
> I still can’t say I think it should be appropriate to do upstream—no real
> info yet on Intel’s problem case--but I hope this explains why the bigger
> hammer (i.e., get Clang involved) doesn’t seem necessary or appropriate.
>
> --paulr
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of
*Reid
> Kleckner via llvm-dev
> *Sent:* Thursday, April 15, 2021 12:59 PM
> *To:* James Y Knight <jyknight at google.com>
> *Cc:* llvm-dev at lists.llvm.org; Liu, Chen3 <chen3.liu at
intel.com>; Luo,
> Yuanke <yuanke.luo at intel.com>; Maslov, Sergey V
<sergey.v.maslov at intel.com
> >
> *Subject:* Re: [llvm-dev] [RFC] [X86] Emit unaligned vector moves on avx
> machine with option control.
>
>
>
> On Wed, Apr 14, 2021 at 11:58 AM James Y Knight via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> What I suspect you *actually* want here is an option to tell Clang not to
> infer load/store alignments based on object types or alignment attributes
> -- instead treating everything as being potentially aligned to 1 unless the
> allocation is seen (e.g. global/local variables). Clang would still need to
> use the usual alignment computation for variable definitions and structure
> layout, but not memory operations. If clang emits "load ... align
1"
> instructions in LLVM IR, the right thing would then happen in the X86
> backend automatically.
>
>
>
> This sounds like the -fmax-type-align flag:
>
> https://clang.llvm.org/docs/UsersManual.html#controlling-code-generation
>
<https://urldefense.com/v3/__https:/clang.llvm.org/docs/UsersManual.html*controlling-code-generation__;Iw!!JmoZiZGBv3RvKRSx!uoBVF33nyuM5lbseJ-XKanIeYhdhHW9yOoxyF7zJ56FjUs8jsfdUcuw4AQ96FRBrmA$>
>
> Explicit alignment attributes are still honored, so some aligned vector
> instructions may be generated. However, the documentation describes
> essentially this exact use case.
>
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