RISC-V: Avoid zero/sign extend for volatile loads. Fix for 97417.

This expands sub-word loads as a zero/sign extended load, followed by
a subreg.  This helps eliminate unnecessary zero/sign extend insns after
the load, particularly for volatiles, but also in some other cases.
Testing shows that it gives consistent code size decreases.

Tested with riscv32-elf rv32imac/ilp32 and riscv64-linux rv64gc/lp064d
builds and checks.  Some -gsplit-stack tests fail with the patch, but
this turns out to be an existing bug with the split-stack support that
I hadn't noticed before.  It isn't a bug in this patch.  Ignoring that
there are no regressions.

Committed.

	gcc/
	PR target/97417
	* config/riscv/riscv-shorten-memrefs.c (pass_shorten_memrefs): Add
	extend parameter to get_si_mem_base_reg declaration.
	(get_si_mem_base_reg): Add extend parameter.  Set it.
	(analyze): Pass extend arg to get_si_mem_base_reg.
	(transform): Likewise.  Use it when rewriting mems.
	* config/riscv/riscv.c (riscv_legitimize_move): Check for subword
	loads and emit sign/zero extending load followed by subreg move.

2 files changed, 49 insertions, 7 deletions

diff --git a/gcc/config/riscv/riscv-shorten-memrefs.c b/gcc/config/riscv/riscv-shorten-memrefs.c
index b1b57f1..3f34065 100644
--- a/gcc/config/riscv/riscv-shorten-memrefs.c
+++ b/gcc/config/riscv/riscv-shorten-memrefs.c
@@ -75,12 +75,19 @@ private:
 
   regno_map * analyze (basic_block bb);
   void transform (regno_map *m, basic_block bb);
-  bool get_si_mem_base_reg (rtx mem, rtx *addr);
+  bool get_si_mem_base_reg (rtx mem, rtx *addr, bool *extend);
 }; // class pass_shorten_memrefs
 
 bool
-pass_shorten_memrefs::get_si_mem_base_reg (rtx mem, rtx *addr)
+pass_shorten_memrefs::get_si_mem_base_reg (rtx mem, rtx *addr, bool *extend)
 {
+  /* Whether it's sign/zero extended.  */
+  if (GET_CODE (mem) == ZERO_EXTEND || GET_CODE (mem) == SIGN_EXTEND)
+    {
+      *extend = true;
+      mem = XEXP (mem, 0);
+    }
+
   if (!MEM_P (mem) || GET_MODE (mem) != SImode)
     return false;
   *addr = XEXP (mem, 0);
@@ -110,7 +117,8 @@ pass_shorten_memrefs::analyze (basic_block bb)
 	{
 	  rtx mem = XEXP (pat, i);
 	  rtx addr;
-	  if (get_si_mem_base_reg (mem, &addr))
+	  bool extend = false;
+	  if (get_si_mem_base_reg (mem, &addr, &extend))
 	    {
 	      HOST_WIDE_INT regno = REGNO (XEXP (addr, 0));
 	      /* Do not count store zero as these cannot be compressed.  */
@@ -150,7 +158,8 @@ pass_shorten_memrefs::transform (regno_map *m, basic_block bb)
 	{
 	  rtx mem = XEXP (pat, i);
 	  rtx addr;
-	  if (get_si_mem_base_reg (mem, &addr))
+	  bool extend = false;
+	  if (get_si_mem_base_reg (mem, &addr, &extend))
 	    {
 	      HOST_WIDE_INT regno = REGNO (XEXP (addr, 0));
 	      /* Do not transform store zero as these cannot be compressed.  */
@@ -161,9 +170,20 @@ pass_shorten_memrefs::transform (regno_map *m, basic_block bb)
 		}
 	      if (m->get_or_insert (regno) > 3)
 		{
-		  addr
-		    = targetm.legitimize_address (addr, addr, GET_MODE (mem));
-		  XEXP (pat, i) = replace_equiv_address (mem, addr);
+		  if (extend)
+		    {
+		      addr
+			= targetm.legitimize_address (addr, addr,
+						      GET_MODE (XEXP (mem, 0)));
+		      XEXP (XEXP (pat, i), 0)
+			= replace_equiv_address (XEXP (mem, 0), addr);
+		    }
+		  else
+		    {
+		      addr = targetm.legitimize_address (addr, addr,
+							 GET_MODE (mem));
+		      XEXP (pat, i) = replace_equiv_address (mem, addr);
+		    }
 		  df_insn_rescan (insn);
 		}
 	    }
diff --git a/gcc/config/riscv/riscv.c b/gcc/config/riscv/riscv.c
index 7d27459..fffd081 100644
--- a/gcc/config/riscv/riscv.c
+++ b/gcc/config/riscv/riscv.c
@@ -1524,6 +1524,28 @@ riscv_legitimize_const_move (machine_mode mode, rtx dest, rtx src)
 bool
 riscv_legitimize_move (machine_mode mode, rtx dest, rtx src)
 {
+  /* Expand 
+       (set (reg:QI target) (mem:QI (address))) 
+     to
+       (set (reg:DI temp) (zero_extend:DI (mem:QI (address))))
+       (set (reg:QI target) (subreg:QI (reg:DI temp) 0))
+     with auto-sign/zero extend.  */
+  if (GET_MODE_CLASS (mode) == MODE_INT
+      && GET_MODE_SIZE (mode) < UNITS_PER_WORD
+      && can_create_pseudo_p ()
+      && MEM_P (src))
+    {
+      rtx temp_reg;
+      int zero_extend_p;
+
+      temp_reg = gen_reg_rtx (word_mode);
+      zero_extend_p = (LOAD_EXTEND_OP (mode) == ZERO_EXTEND);
+      emit_insn (gen_extend_insn (temp_reg, src, word_mode, mode, 
+				  zero_extend_p));
+      riscv_emit_move (dest, gen_lowpart (mode, temp_reg));
+      return true;
+    }
+
   if (!register_operand (dest, mode) && !reg_or_0_operand (src, mode))
     {
       rtx reg;