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* [PATCH] Preserve SUBREG_PROMOTED_VAR_P on (extend:HI (subreg/s:QI (reg:SI)))
@ 2021-08-29  7:46 Roger Sayle
  2021-08-29 20:59 ` Jeff Law
  2021-08-31 12:32 ` Christophe LYON
  0 siblings, 2 replies; 6+ messages in thread
From: Roger Sayle @ 2021-08-29  7:46 UTC (permalink / raw)
  To: 'GCC Patches'

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SUBREG_PROMOTED_VAR_P is a mechanism for tracking that a partial subreg
is correctly zero-extended or sign-extended in the parent register.  For
example, the RTL (subreg/s/v:QI (reg/v:SI 23 [ x ]) 0) indicates that the
byte x is zero extended in reg:SI 23, which is useful for optimization.
An example is that zero extending the above QImode value to HImode can
simply use a wider subreg, i.e. (subreg:HI (reg/v:SI 23 [ x ]) 0).

This patch addresses the oversight/missed optimization opportunity that
the new HImode subreg above should retain its SUBREG_PROMOTED_VAR_P
annotation as its value is guaranteed to be correctly extended in the
SImode parent.  The code below to preserve SUBREG_PROMOTED_VAR_P is already
present in the middle-end (e.g. simplify-rtx.c:7232-7242) but missing
from one or two (precisely three) places that (accidentally) strip it.

Whilst there I also added another optimization.  If we need to extend
the above QImode value beyond the SImode register holding it, say to
DImode, we can eliminate the SUBREG and simply extend from the SImode
register to DImode.

This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check" with no new failures, and on a cross-compiler to
nvptx-none, where the function "long foo(char x) { return x; }" now
requires one less instruction.

OK for mainline?


2021-08-29  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* expr.c (convert_modes): Preserve SUBREG_PROMOTED_VAR_P when
	creating a (wider) partial subreg from a SUBREG_PROMOTED_VAR_P
	subreg.
	* simplify-rtx.c (simplify_unary_operation_1) [SIGN_EXTEND]:
	Likewise, preserve SUBREG_PROMOTED_VAR_P when creating a (wider)
	partial subreg from a SUBREG_PROMOTED_VAR_P subreg.  Generate
	SIGN_EXTEND of the SUBREG_REG when a subreg would be paradoxical.
	[ZERO_EXTEND]: Likewise, preserve SUBREG_PROMOTED_VAR_P when
	creating a (wider) partial subreg from a SUBREG_PROMOTED_VAR_P
	subreg.  Generate ZERO_EXTEND of the SUBREG_REG when a subreg
	would be paradoxical.

Roger
--


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diff --git a/gcc/expr.c b/gcc/expr.c
index 096c031..5dd98a9 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -688,7 +688,24 @@ convert_modes (machine_mode mode, machine_mode oldmode, rtx x, int unsignedp)
       && (GET_MODE_PRECISION (subreg_promoted_mode (x))
 	  >= GET_MODE_PRECISION (int_mode))
       && SUBREG_CHECK_PROMOTED_SIGN (x, unsignedp))
-    x = gen_lowpart (int_mode, SUBREG_REG (x));
+    {
+      scalar_int_mode int_orig_mode;
+      machine_mode orig_mode = GET_MODE (x);
+      x = gen_lowpart (int_mode, SUBREG_REG (x));
+
+      /* Preserve SUBREG_PROMOTED_VAR_P if the new mode is wider than
+	 the original mode, but narrower than the inner mode.  */
+      if (GET_CODE (x) == SUBREG
+	  && GET_MODE_PRECISION (subreg_promoted_mode (x))
+	     > GET_MODE_PRECISION (int_mode)
+	  && is_a <scalar_int_mode> (orig_mode, &int_orig_mode)
+	  && GET_MODE_PRECISION (int_mode)
+	     > GET_MODE_PRECISION (int_orig_mode))
+	{
+	  SUBREG_PROMOTED_VAR_P (x) = 1;
+	  SUBREG_PROMOTED_SET (x, unsignedp);
+	}
+    }
 
   if (GET_MODE (x) != VOIDmode)
     oldmode = GET_MODE (x);
diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index c81e27e..a75f6b2 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -1511,12 +1511,28 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
 	 target mode is the same as the variable's promotion.  */
       if (GET_CODE (op) == SUBREG
 	  && SUBREG_PROMOTED_VAR_P (op)
-	  && SUBREG_PROMOTED_SIGNED_P (op)
-	  && !paradoxical_subreg_p (mode, GET_MODE (SUBREG_REG (op))))
+	  && SUBREG_PROMOTED_SIGNED_P (op))
 	{
-	  temp = rtl_hooks.gen_lowpart_no_emit (mode, SUBREG_REG (op));
-	  if (temp)
-	    return temp;
+	  rtx subreg = SUBREG_REG (op);
+	  machine_mode subreg_mode = GET_MODE (subreg);
+	  if (!paradoxical_subreg_p (mode, subreg_mode))
+	    {
+	      temp = rtl_hooks.gen_lowpart_no_emit (mode, subreg);
+	      if (temp)
+		{
+		  /* Preserve SUBREG_PROMOTED_VAR_P.  */
+		  if (partial_subreg_p (temp))
+		    {
+		      SUBREG_PROMOTED_VAR_P (temp) = 1;
+		      SUBREG_PROMOTED_SET (temp, 1);
+		    }
+		  return temp;
+		}
+	    }
+	  else
+	    /* Sign-extending a sign-extended subreg.  */
+	    return simplify_gen_unary (SIGN_EXTEND, mode,
+				       subreg, subreg_mode);
 	}
 
       /* (sign_extend:M (sign_extend:N <X>)) is (sign_extend:M <X>).
@@ -1630,12 +1646,28 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
 	 target mode is the same as the variable's promotion.  */
       if (GET_CODE (op) == SUBREG
 	  && SUBREG_PROMOTED_VAR_P (op)
-	  && SUBREG_PROMOTED_UNSIGNED_P (op)
-	  && !paradoxical_subreg_p (mode, GET_MODE (SUBREG_REG (op))))
+	  && SUBREG_PROMOTED_UNSIGNED_P (op))
 	{
-	  temp = rtl_hooks.gen_lowpart_no_emit (mode, SUBREG_REG (op));
-	  if (temp)
-	    return temp;
+	  rtx subreg = SUBREG_REG (op);
+	  machine_mode subreg_mode = GET_MODE (subreg);
+	  if (!paradoxical_subreg_p (mode, subreg_mode))
+	    {
+	      temp = rtl_hooks.gen_lowpart_no_emit (mode, subreg);
+	      if (temp)
+		{
+		  /* Preserve SUBREG_PROMOTED_VAR_P.  */
+		  if (partial_subreg_p (temp))
+		    {
+		      SUBREG_PROMOTED_VAR_P (temp) = 1;
+		      SUBREG_PROMOTED_SET (temp, 0);
+		    }
+		  return temp;
+		}
+	    }
+	  else
+	    /* Zero-extending a zero-extended subreg.  */
+	    return simplify_gen_unary (ZERO_EXTEND, mode,
+				       subreg, subreg_mode);
 	}
 
       /* Extending a widening multiplication should be canonicalized to

^ permalink raw reply	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2021-08-31 21:00 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-08-29  7:46 [PATCH] Preserve SUBREG_PROMOTED_VAR_P on (extend:HI (subreg/s:QI (reg:SI))) Roger Sayle
2021-08-29 20:59 ` Jeff Law
2021-08-31 12:32 ` Christophe LYON
2021-08-31 13:24   ` Roger Sayle
2021-08-31 15:30     ` Jeff Law
2021-08-31 21:00     ` Rainer Orth

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