retry zero-call-used-regs from zeroed regs

default_zero_call_used_regs currently requires all potentially zeroed
registers to offer a move opcode that accepts zero as an operand.

This is not the case e.g. for ARM's r12/ip in Thumb mode, and it was
not the case of FP registers on AArch64 as of GCC 10.

This patch introduces a fallback strategy to zero out registers,
copying from registers that have already been zeroed.  Adjacent
sources to make up wider modes are also supported.

This does not guarantee that there will be some zeroed-out register to
use as the source, but it expands the cases in which the default
implementation works out of the box.


for  gcc/ChangeLog

	* targhooks.c (default_zero_call_used_regs): Retry using
	successfully-zeroed registers as sources.

1 files changed, 86 insertions, 7 deletions

diff --git a/gcc/targhooks.c b/gcc/targhooks.c
index 2e0fdb7..1947ef2 100644
--- a/gcc/targhooks.c
+++ b/gcc/targhooks.c
@@ -1001,6 +1001,13 @@ default_zero_call_used_regs (HARD_REG_SET need_zeroed_hardregs)
 {
   gcc_assert (!hard_reg_set_empty_p (need_zeroed_hardregs));
 
+  HARD_REG_SET failed;
+  CLEAR_HARD_REG_SET (failed);
+  bool progress = false;
+
+  /* First, try to zero each register in need_zeroed_hardregs by
+     loading a zero into it, taking note of any failures in
+     FAILED.  */
   for (unsigned int regno = 0; regno < FIRST_PSEUDO_REGISTER; regno++)
     if (TEST_HARD_REG_BIT (need_zeroed_hardregs, regno))
       {
@@ -1010,16 +1017,88 @@ default_zero_call_used_regs (HARD_REG_SET need_zeroed_hardregs)
 	rtx_insn *insn = emit_move_insn (regno_reg_rtx[regno], zero);
 	if (!valid_insn_p (insn))
 	  {
-	    static bool issued_error;
-	    if (!issued_error)
-	      {
-		issued_error = true;
-		sorry ("%qs not supported on this target",
-			"-fzero-call-used-regs");
-	      }
+	    SET_HARD_REG_BIT (failed, regno);
 	    delete_insns_since (last_insn);
 	  }
+	else
+	  progress = true;
       }
+
+  /* Now retry with copies from zeroed registers, as long as we've
+     made some PROGRESS, and registers remain to be zeroed in
+     FAILED.  */
+  while (progress && !hard_reg_set_empty_p (failed))
+    {
+      HARD_REG_SET retrying = failed;
+
+      CLEAR_HARD_REG_SET (failed);
+      progress = false;
+
+      for (unsigned int regno = 0; regno < FIRST_PSEUDO_REGISTER; regno++)
+	if (TEST_HARD_REG_BIT (retrying, regno))
+	  {
+	    machine_mode mode = GET_MODE (regno_reg_rtx[regno]);
+	    bool success = false;
+	    /* Look for a source.  */
+	    for (unsigned int src = 0; src < FIRST_PSEUDO_REGISTER; src++)
+	      {
+		/* If SRC hasn't been zeroed (yet?), skip it.  */
+		if (! TEST_HARD_REG_BIT (need_zeroed_hardregs, src))
+		  continue;
+		if (TEST_HARD_REG_BIT (retrying, src))
+		  continue;
+
+		/* Check that SRC can hold MODE, and that any other
+		   registers needed to hold MODE in SRC have also been
+		   zeroed.  */
+		if (!targetm.hard_regno_mode_ok (src, mode))
+		  continue;
+		unsigned n = targetm.hard_regno_nregs (src, mode);
+		bool ok = true;
+		for (unsigned i = 1; ok && i < n; i++)
+		  ok = (TEST_HARD_REG_BIT (need_zeroed_hardregs, src + i)
+			&& !TEST_HARD_REG_BIT (retrying, src + i));
+		if (!ok)
+		  continue;
+
+		/* SRC is usable, try to copy from it.  */
+		rtx_insn *last_insn = get_last_insn ();
+		rtx zsrc = gen_rtx_REG (mode, src);
+		rtx_insn *insn = emit_move_insn (regno_reg_rtx[regno], zsrc);
+		if (!valid_insn_p (insn))
+		  /* It didn't work, remove any inserts.  We'll look
+		     for another SRC.  */
+		  delete_insns_since (last_insn);
+		else
+		  {
+		    /* We're done for REGNO.  */
+		    success = true;
+		    break;
+		  }
+	      }
+
+	    /* If nothing worked for REGNO this round, marked it to be
+	       retried if we get another round.  */
+	    if (!success)
+	      SET_HARD_REG_BIT (failed, regno);
+	    else
+	      /* Take note so as to enable another round if needed.  */
+	      progress = true;
+	  }
+    }
+
+  /* If any register remained, report it.  */
+  if (!progress)
+    {
+      static bool issued_error;
+      if (!issued_error)
+	{
+	  issued_error = true;
+	  sorry ("%qs not supported on this target",
+		 "-fzero-call-used-regs");
+	}
+    }
+
   return need_zeroed_hardregs;
 }