On Fri, Apr 22, 2016 at 01:22:51PM +0000, Wilco Dijkstra wrote: > Improve modes_tieable by returning true in more cases: allow scalar access > within vectors without requiring an extra move. Removing these moves helps > the register allocator in deciding whether to use integer or FP registers on > operations that can be done on both. This saves about 100 instructions in the > gcc.target/aarch64 tests. >
[snip] > diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c > index abc864c..6e921f0 100644 > --- a/gcc/config/aarch64/aarch64.c > +++ b/gcc/config/aarch64/aarch64.c > @@ -12294,7 +12294,14 @@ aarch64_reverse_mask (enum machine_mode mode) > return force_reg (V16QImode, mask); > } > > -/* Implement MODES_TIEABLE_P. */ > +/* Implement MODES_TIEABLE_P. In principle we should always return true. > + However due to issues with register allocation it is preferable to avoid > + tieing integer scalar and FP scalar modes. Executing integer operations > + in general registers is better than treating them as scalar vector > + operations. This reduces latency and avoids redundant int<->FP moves. > + So tie modes if they are either the same class, or vector modes with > + other vector modes, vector structs or any scalar mode. > +*/ */ shouldn't be on the newline, just "[...] scalar mode. */" It would be handy if you could raise something in bugzilla for the register allocator deficiency. > bool > aarch64_modes_tieable_p (machine_mode mode1, machine_mode mode2) > @@ -12305,9 +12312,12 @@ aarch64_modes_tieable_p (machine_mode mode1, > machine_mode mode2) > /* We specifically want to allow elements of "structure" modes to > be tieable to the structure. This more general condition allows > other rarer situations too. */ > - if (TARGET_SIMD > - && aarch64_vector_mode_p (mode1) > - && aarch64_vector_mode_p (mode2)) > + if (aarch64_vector_mode_p (mode1) && aarch64_vector_mode_p (mode2)) > + return true; This relaxes the TARGET_SIMD check that would have prevented OImode/CImode/XImode ties when !TARGET_SIMD. What's the reasoning behind that? > + /* Also allow any scalar modes with vectors. */ > + if (aarch64_vector_mode_supported_p (mode1) > + || aarch64_vector_mode_supported_p (mode2)) > return true; Does this always hold? It seems like you might need to be more restrictive with what we allow to avoid ties with some of the more obscure modes (V4DF etc.). Thanks, James
