[PATCH][ARM] Improve the instruction selection of vector loads.

[Jim Grosbach]

LGTM, thanks!

-Jim
On Jul 3, 2013, at 10:09 AM, Quentin Colombet <qcolombet at apple.com> wrote:

> Ping?
> 
> -Quentin
> 
> On Jul 1, 2013, at 5:42 PM, Quentin Colombet <qcolombet at apple.com> wrote:
> 
>> Hi,
>> 
>> Here is a patch to improve the instruction selection of vector loads on ARM.
>> Thanks for your review.
>> 
>> ** Problematic **
>> In the ARM back-end, build_vector nodes are lowered to a target specific build_vector that uses floating point type[1]. 
>> This works well, unless the inserted bitcasts survive until instruction selection. In that case, they incur moves between integer unit and floating point unit that may result in inefficient code.
>> 
>> In other words, this conversion may introduce artificial dependencies when the code leading to the build vector cannot be completed with a floating point type.
>> 
>> In particular, this happens when loads are not aligned.
>> 
>> In that case, the compiler generates general purpose loads and creates the floating point vector from them, instead of directly using the vector unit.
>> 
>> <rdar://problem/14170854>
>> 
>> [1] The rational is that floating point registers are aliases of vector registers.
>> 
>> ** Motivating Example **
>> The attached motivating_example.ll demonstrates that (also part of a test case in the proposed patch).
>> 
>> To reproduce:
>> llc -O3 -mtriple thumbv7-apple-ios3 motivating_example.ll -o -
>> 	ldr	r0, [r1]
>> 	ldr	r1, [r2]
>> 	vmov	s1, r1
>> 	vmov	s0, r0
>> Here each ldr, vmov sequences could have been replaced by a simple vld1.32.
>> 
>> ** Proposed Solution **
>> Use a vector friendly sequence of code when the inserted bitcasts to floating point survived DAGCombine.
>> 
>> Thanks for Eli Friedman for the direction!
>> 
>> This is done by a target specific DAGCombine that changes the target specific build_vector into a sequence of insert_vector_elt that get rid of the bitcasts.
>> 
>> Thanks for your review.
>> 
>> Cheers,
>> 
>> -Quentin
>> <ARMISelLowering.patch> <motivating_example.ll> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> 
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20130703/14f43ace/attachment.html>