[PATCH] D31711: [GlobalISel] LegalizerInfo: Enable legalization of vector types with non-power-of-2 number of elements

[Ahmed Bougacha via Phabricator via llvm-commits]

ab accepted this revision.
ab added a comment.
This revision is now accepted and ready to land.

LGTM after removing the whole check and adding a good explanation in the commit message.

Can you add a simple vector test in test/CodeGen/AArch64/GlobalISel/arm64-fallback.ll?  It already has a scalar test (that shows the change is OK if it continues reporting a fallback)



================
Comment at: lib/CodeGen/GlobalISel/LegalizerInfo.cpp:78-81
+  LLT Ty = Aspect.Type;
+  unsigned SizeInBits =
+      Ty.isVector() ? Ty.getElementType().getSizeInBits() : Ty.getSizeInBits();
+  if (!isPowerOf2_64(SizeInBits))
----------------
kristof.beyls wrote:
> volkan wrote:
> > kristof.beyls wrote:
> > > I was confused by the summary on this patch - I thought you wanted to make sure vector types like <3 x s32> definitely wouldn't be legalized and thought the existing code could sometimes legalize them. It then started doing the following train-of-thought:
> > > """
> > > For vector types, getSizeInBits() returns the "number of elements" times "element size in bits".
> > > That result can only be a power of 2 if both "number of elements" and "element size in bits" are both a power of 2.
> > > So, the effect of this change would be to allow more vector data types, like <3 x s32> to be legalized."
> > > 
> > > So, in summary, by the time this gets committed, I think the commit message must be clearer, and e.g. state "Enable legalization of vector types with non-power-of-2 number of elements".
> > > 
> > > This patch clearly is missing tests.
> > > 
> > > I was confused by the summary on this patch - I thought you wanted to make sure vector types like <3 x s32> definitely wouldn't be legalized and thought the existing code could sometimes legalize them. It then started doing the following train-of-thought:
> > > """
> > > For vector types, getSizeInBits() returns the "number of elements" times "element size in bits".
> > > That result can only be a power of 2 if both "number of elements" and "element size in bits" are both a power of 2.
> > > So, the effect of this change would be to allow more vector data types, like <3 x s32> to be legalized."
> > > 
> > > So, in summary, by the time this gets committed, I think the commit message must be clearer, and e.g. state "Enable legalization of vector types with non-power-of-2 number of elements".
> > > 
> > 
> > Yes, it seems I mixed the intent and the patch. I'll update the title.
> > 
> > > This patch clearly is missing tests.
> > 
> > I didn’t add a test case because there wasn't any legal operations on vector types with non-power-of-2 number of elements, but I just realized I can use G_MERGE_VALUES.
> > 
> > Thank you,
> > Volkan
> > 
> Thanks for those changes.
> 
> I'm wondering now if we shouldn't just completely drop the following:
> ```
>   if (!isPowerOf2_64(SizeInBits))
>     return std::make_pair(Unsupported, LLT());
> ```
> and just let the fall-back mechanism catch the many/all cases where non-power-of-2-sized types aren't supported.
> Removing that if statement alltogether makes it a bit easier to also work towards legalizing non-power-of-2 scalar types.
> IIRC, Tim introduced this, so I guess he has the best view on the tradeoffs here.
> 
> @t.p.northover : Do you have any thoughts on this?
I agree, we should probably drop the check completely.

I think it was more of a defensive way to avoid even considering non-power-of-2 types elsewhere.  But looking around, it seems we're robust enough to fail legalization properly, so it's fine to do that.


https://reviews.llvm.org/D31711