Re: [PATCH 2/2] arm: lib: xor-neon: disable clang vectorization

quoted

On Tue, 10 Nov 2020, Nick Desaulniers [off-list ref] wrote:
On Tue, Nov 10, 2020 at 3:54 PM Adrian Ratiu [off-list ref] wrote:
On Tue, 10 Nov 2020, Nick Desaulniers [off-list ref] wrote:
On Mon, Nov 9, 2020 at 11:51 AM Adrian Ratiu [off-list ref] wrote:
On Fri, 06 Nov 2020, Nick Desaulniers [off-list ref] wrote:
+#pragma clang loop vectorize(enable)         do {                 p1[0] ^= p2[0] ^ p3[0] ^ p4[0] ^ p5[0];                 p1[1] ^= p2[1] ^ p3[1] ^ p4[1] ^ p5[1]; ``` seems to generate the vectorized code.

Why don't we find a way to make those pragma's more toolchain portable, rather than open coding them like I have above rather than this series?
Hi again Nick,

How did you verify the above pragmas generate correct vectorized code?  Have you tested this specific use case?
I read the disassembly before and after my suggested use of pragmas; look for vld/vstr.  You can also add -Rpass-missed=loop-vectorize to CFLAGS_xor-neon.o in arch/arm/lib/Makefile and rebuild arch/arm/lib/xor-neon.o with CONFIG_BTRFS enabled.

I'm asking because overrulling the cost model might not be enough, the only thing I can confirm is that the generated code is changed, but not that it is correct in any way. The object disasm also looks weird, but I don't have enough knowledge to start debugging what's happening within LLVM/Clang itself.
It doesn't "look weird" to me. The loop is versioned based on a comparison whether the parameters alias or not. There's a non-vectorized version if the parameters are equal or close enough to overlap.  There's another version of the loop that's vectorized.  If you want just the vectorized version, then you have to mark the parameters as __restrict qualified, then check that all callers are ok with that.
Thank you for the explanation, that does make sense now. I'm just a compiler optimization noob, sorry. All your help is much appreciated.
Don't worry about it; you'll get the hang of it in no time, just stick with it.

I also get some new warnings with your code [1], besides the previously 'vectorization was possible but not beneficial' which is still present. It is quite funny because these two warnings seem to contradict themselves. :)
From which compiler?  ``` $ clang -Wpass-failed=transform-warning -c -x c /dev/null warning: unknown warning option '-Wpass-failed=transform-warning'; did you mean '-Wprofile-instr-missing'? [-Wunknown-warning-option] ```
I'm using Clang 10.0.1-1 from the Arch Linux repo.

In the LLVM sources that transform-warning appears to be documented under llvm-10.0.1.src/docs/Passes.rst:1227:-transform-warning

Here's a build log: http://ix.io/2DIc

I always get those warnings with the pragma change you suggested, even on clean builds on latest linux-next.

I looked at the Arch PKGBUILD and they don't appear to do anything special other than patching to enable SSP and PIE by default (eg llvm bug 13410).
Ah, custom builds of LLVM.  Grepping for transform-warning in LLVM's sources, I can indeed see such a pass. I'm curious whether Arch is turning on that pass by default or if you manually enabled -Wpass-failed=transform-warning in the Makefile?  Maybe I need to do an assertions enabled build of LLVM or a debug build. Reading through llvm/docs/Passes.rst and llvm/docs/TransformMetadata.rst, it sounds like this should be triggered when a "forced optimization has failed."  So I wonder what's the missing variable between it working for me, vs warning for you?
I did not build clang myself, just did "pacman -S clang" to get the official distro binary package. Here's the PKGBUILD they used, I'm sending the commit link because recently clang 11 was upgraded to.

I also tested clang 11.0.0 where I get the same warnings / remarks.

https://github.com/archlinux/svntogit-packages/blob/8ff1bb4e4be5c6e5bede60c6b259a89f0cee6e6a/trunk/PKGBUILD

Godbolt seems to agree with me here: https://godbolt.org/z/Wf6YKv.  Maybe related to the "New Pass Manager" ... digging into that...

The pragma is clang specific, hence my recommendation to wrap it in an #ifdef __clang__.
Yes, I understand that. :)

At this point I do not trust the compiler and am inclined to do
Nonsense.

like was done for GCC when it was broken: disable the optimization and warn users to upgrade after the compiler is fixed and confirmed to work.

If you agree I can send a v2 with this and also drop the GCC pragma as Arvind and Ard suggested.
If you resend "this" as in 2/2, I will NACK it.  There's nothing wrong with the cost model; it's saying there's little point in generating the vectorized version because you're still going to need a non-vectorized loop version anyways. Claiming there is a compiler bug here is dubious just because the cost models between two compilers differ slightly.
Ok, so that "remark" from the compiler is safe to ignore.
Are you always seeing it when building with the pragma's added, no change to CFLAGS_xor-neon.o in arch/arm/lib/Makefile?
No, I have to modify CFLAGS_xor-neon.o to see the remarks. If I do a build with just the pragma change I only always get the warnings, not remarks.

Here's a more complete log with -Rpass-missed='.*' in the Makefile, maybe the other remarks in there will help shed some light.

http://ix.io/2DMl

Resend the patch removing the warning, remove the GCC pragma, but if you want to change anything here for Clang, use `#pragma clang loop vectorize(enable)` wrapped in an `#ifdef __clang__`.
Thanks for making the NACK clear, so the way forward is to either use the pragma if I can figure out the new 'loop not vectorized' warning (which might also be a red herring) or just leave Clang as is. :)
Yes, though additionally Arvind points out that this code is kind of curious if there was overlap; maybe the parameters should just be restrict-qualified.
For now I think I'll just re-send the GCC changes and leave the Clang optimization as is, until we better understand what's happening and what's the best way to enable it.

Kind regards,
Adrian

[1]
./include/asm-generic/xor.h:11:1: warning: loop not vectorized:
the optimizer was unable to perform the requested transformation;
the transformation might be disabled or specified as part of an
unsupported transformation ordering
[-Wpass-failed=transform-warning] xor_8regs_2(unsigned long bytes,
unsigned long *p1, unsigned long *p2)
-- 
Thanks,
~Nick Desaulniers
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