[PATCH] D42424: [InstCombine] Allow common type conversions to i8/i16

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

I added a different transform that is guarded by shouldChangeType() at https://reviews.llvm.org/rL323437 .
The tests added in that patch will be affected by this patch, so please rebase. I don't have any other suggestions, so if there are no other comments, let's try it?

I think the difference in AArch64 code with this patch for the examples in PR35792 would be:

  $ ./opt  35792.ll -S | ./llc -o - -mtriple=aarch64
  julia_a_62828:                          // @julia_a_62828
  	mov	w8, w0
  	sub	w9, w0, #1              // =1
  	and	x0, x9, x8
  	ret
  bad:                                    // @bad
  	orr	w8, wzr, #0xffff
  	add	w8, w0, w8
  	and	w0, w0, w8
  	ret

vs. after this patch:

  $ ./opt -instcombine  35792.ll -S | ./llc -o - -mtriple=aarch64
  julia_a_62828:                          // @julia_a_62828
  	sub	w8, w0, #1              // =1
  	and	w0, w8, w0
  	ret
  bad:                                    // @bad
  	sub	w8, w0, #1              // =1
  	and	w0, w8, w0
  	ret



================
Comment at: lib/Transforms/InstCombine/InstructionCombining.cpp:147
 /// because i1 is a fundamental type in IR, and there are many specialized
-/// optimizations for i1 types.
+/// optimizations for i1 types. Widths or 8, 16 or 32 are equally treated as
+/// legal to convert to, in order to open up more combining opportunities.
----------------
or -> of ?


https://reviews.llvm.org/D42424