PATCH: PR rtl-optimization/50696: [x32] Unnecessary lea

H.J. Lu hjl.tools@gmail.com
Thu Oct 13 22:42:00 GMT 2011


On Thu, Oct 13, 2011 at 2:45 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Thu, Oct 13, 2011 at 2:30 PM, Richard Kenner
> <kenner@vlsi1.ultra.nyu.edu> wrote:
>>> It is because mask 0xffffffff is optimized to 0xfffffffc by keeping track
>>> of non-zero bits in registers and the above code doesn't take that
>>> into account.
>>
>> Then I'd suggest modifying that code so that it does rather than
>> essentially duplicating it.  But I'd recommend running some
>> performance tests to verify that you're not pessimizing things when
>> you do that: this stuff can be very tricky and you want to make sure
>> that you're not converting something like (and X 3) into a bit
>> extraction unnecessarily.
>>
>
> But the current code converts (and X 3) into a bit extraction
> since ((i = exact_log2 (UINTVAL (XEXP (x, 1)) + 1)) >= 0) is true
> when UINTVAL (XEXP (x, 1))  == 3.  Should we do it or not?
>

I am testing this patch.  The difference is it checks nonzero
bits of the first operand.

-- 
H.J.
--

diff --git a/gcc/combine.c b/gcc/combine.c
index 6c3b17c..598dee3 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -7739,16 +7739,6 @@ make_compound_operation (rtx x, enum rtx_code in_code)
 				     XEXP (XEXP (x, 0), 1)));
 	}

-      /* If the constant is one less than a power of two, this might be
-	 representable by an extraction even if no shift is present.
-	 If it doesn't end up being a ZERO_EXTEND, we will ignore it unless
-	 we are in a COMPARE.  */
-      else if ((i = exact_log2 (UINTVAL (XEXP (x, 1)) + 1)) >= 0)
-	new_rtx = make_extraction (mode,
-			       make_compound_operation (XEXP (x, 0),
-							next_code),
-			       0, NULL_RTX, i, 1, 0, in_code == COMPARE);
-
       /* If we are in a comparison and this is an AND with a power of two,
 	 convert this into the appropriate bit extract.  */
       else if (in_code == COMPARE
@@ -7758,6 +7748,23 @@ make_compound_operation (rtx x, enum rtx_code in_code)
 							next_code),
 			       i, NULL_RTX, 1, 1, 0, 1);

+      /* If we are (and (OP) M) and M is an extraction mask, this is an
+	 extraction.  */
+      else
+	{
+	  unsigned HOST_WIDE_INT nonzero =
+	    nonzero_bits (XEXP (x, 0), GET_MODE (XEXP (x, 0)));
+	  unsigned HOST_WIDE_INT mask = UINTVAL (XEXP (x, 1));
+	  unsigned HOST_WIDE_INT len = ceil_log2 (mask);
+	  if ((nonzero & (((unsigned HOST_WIDE_INT) 1 << len) - 1))
+	      == (nonzero & mask))
+	    {
+	      new_rtx = make_compound_operation (XEXP (x, 0), next_code);
+	      new_rtx = make_extraction (mode, new_rtx, 0, NULL_RTX,
+					 len, 1, 0, in_code == COMPARE);
+	    }
+	}
+
       break;

     case LSHIFTRT:



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