[PATCH 1/5] [amdgcn] Use first lane of v1 for zero constant

[Kwok Cheung Yeung]

GCN 5 has commonly-used global memory instructions that specify the 
address as [SGPR address] + [VGPR offset] + [constant offset], and we 
often want the VGPR offset to be zero, so v0 is currently reserved for 
that purpose.

However, v1 contains [0, 1, 2..., 63], and as we only use the first lane 
of the VGPR for the offset (the instructions actually work on vectors of 
addresses, but we only employ them in single-lane mode for all memory 
accesses except for explicit scatter-gather instructions), v1 can be 
used in place of v0, freeing v0 for other purposes.

Okay for trunk?

Kwok


2019-11-14  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* config/gcn/gcn.c (gcn_expand_prologue): Remove initialization and
	prologue use of v0.
	(print_operand_address): Use v1 for zero vector offset.
---
  gcc/config/gcn/gcn.c | 17 +++--------------
  1 file changed, 3 insertions(+), 14 deletions(-)

diff --git a/gcc/config/gcn/gcn.c b/gcc/config/gcn/gcn.c
index 1a69737..2c08771 100644
--- a/gcc/config/gcn/gcn.c
+++ b/gcc/config/gcn/gcn.c
@@ -2799,15 +2799,6 @@ gcn_expand_prologue ()
  				     cfun->machine->args.
  				     reg[PRIVATE_SEGMENT_WAVE_OFFSET_ARG]);

-      if (TARGET_GCN5_PLUS)
-	{
-	  /* v0 is reserved for constant zero so that "global"
-	     memory instructions can have a nul-offset without
-	     causing reloads.  */
-	  emit_insn (gen_vec_duplicatev64si
-		     (gen_rtx_REG (V64SImode, VGPR_REGNO (0)), const0_rtx));
-	}
-
        if (cfun->machine->args.requested & (1 << FLAT_SCRATCH_INIT_ARG))
  	{
  	  rtx fs_init_lo =
@@ -2866,8 +2857,6 @@ gcn_expand_prologue ()
  		  gen_int_mode (LDS_SIZE, SImode));

    emit_insn (gen_prologue_use (gen_rtx_REG (SImode, M0_REG)));
-  if (TARGET_GCN5_PLUS)
-    emit_insn (gen_prologue_use (gen_rtx_REG (SImode, VGPR_REGNO (0))));

    if (cfun && cfun->machine && !cfun->machine->normal_function && 
flag_openmp)
      {
@@ -5324,9 +5313,9 @@ print_operand_address (FILE *file, rtx mem)
  	      /* The assembler requires a 64-bit VGPR pair here, even though
  	         the offset should be only 32-bit.  */
  	      if (vgpr_offset == NULL_RTX)
-		/* In this case, the vector offset is zero, so we use v0,
-		   which is initialized by the kernel prologue to zero.  */
-		fprintf (file, "v[0:1]");
+		/* In this case, the vector offset is zero, so we use the first
+		   lane of v1, which is initialized to zero.  */
+		fprintf (file, "v[1:2]");
  	      else if (REG_P (vgpr_offset)
  		       && VGPR_REGNO_P (REGNO (vgpr_offset)))
  		{
-- 
2.8.1