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.

A typical example:

	orr     v1.8b, v0.8b, v1.8b
	fmov    x0, d0
	fmov    x1, d1
	add     x0, x1, x0
	ins     v0.d[0], x0
	orr     v0.8b, v1.8b, v0.8b

after:

	orr     v1.8b, v0.8b, v1.8b
	add     d0, d1, d0
	orr     v0.8b, v1.8b, v0.8b

    gcc/
	* config/aarch64/aarch64.c (aarch64_modes_tieable_p):
	Allow scalar/single vector modes to be tieable.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@237597 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 14 insertions, 4 deletions

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 4497002707e..46c917a17bc 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -12819,7 +12819,14 @@ 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.
+*/
 
 bool
 aarch64_modes_tieable_p (machine_mode mode1, machine_mode mode2)
@@ -12830,9 +12837,12 @@ 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;
+
+  /* Also allow any scalar modes with vectors.  */
+  if (aarch64_vector_mode_supported_p (mode1)
+      || aarch64_vector_mode_supported_p (mode2))
     return true;
 
   return false;