Re: [patch][simplify-rtx] Fix 16-bit -> 64-bit multiply and accumulate

[Andrew Stubbs <ams@codesourcery.com>]

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On 24/05/11 20:35, Joseph S. Myers wrote:
> On Tue, 24 May 2011, Andrew Stubbs wrote:
>
>> I've created this new, simpler patch that converts
>>
>>    (extend (mult a b))
>>
>> into
>>
>>    (mult (extend a) (extend b))
>>
>> regardless of what 'a' and 'b' might be. (These are then simplified and
>> superfluous extends removed, of course.)
>
> Are there some missing conditions here?  The two aren't equivalent in
> general - (extend:SI (mult:HI a b)) multiplies the HImode values in HImode
> (with modulo arithmetic on overflow) before extending the possibly wrapped
> result to SImode.

Ok, I've now modified my patch to prevent it widening regular multiplies.

It now converts

    (extend (mult (extend a) (extend b)))

to

    (mult (newextend a) (newextend b))

But I also have it convert

    (extend (mult (shift a) (extend b)))

to

    (mult (newextend (shift a)) (newextend b))

The latter case is to catch widening multiplies that extract a subreg 
using a shift. I don't understand why it doesn't just use subreg in the 
first place, but apparently it doesn't, and changing it to do that would 
no doubt break many existing machine descriptions.

The latter case also happens to catch cases where an extend is 
represented by (ashiftrt (ashift x)), which is nice.

I know that, potentially, not all shifted operands are going to be 
widening multiplies, but I *think* this should be safe because other 
random shift values are unlikely to match a real widening mult 
instruction (and if they do then the code would already be broken). If 
somebody knows a reason why this isn't safe then I think I'm going to 
need some help figuring out what conditions to use.

OK?

Andrew

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2011-05-25  Bernd Schmidt  <bernds@codesourcery.com>
	    Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* simplify-rtx.c (simplify_unary_operation_1): Canonicalize widening
	multiplies.
	* doc/md.texi (Canonicalization of Instructions): Document widening
	multiply canonicalization.

	gcc/testsuite/
	* gcc.target/arm/mla-2.c: New test.

--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -5840,6 +5840,21 @@ Equality comparisons of a group of bits (usually a single bit) with zero
 will be written using @code{zero_extract} rather than the equivalent
 @code{and} or @code{sign_extract} operations.
 
+@cindex @code{mult}, canonicalization of
+@item
+@code{(sign_extend:@var{m1} (mult:@var{m2} (sign_extend:@var{m2} @var{x})
+(sign_extend:@var{m2} @var{y})))} is converted to @code{(mult:@var{m1}
+(sign_extend:@var{m1} @var{x}) (sign_extend:@var{m1} @var{y}))}, and likewise
+for @code{zero_extend}.
+
+@item
+@code{(sign_extend:@var{m1} (mult:@var{m2} (ashiftrt:@var{m2}
+@var{x} @var{s}) (sign_extend:@var{m2} @var{y})))} is converted to
+@code{(mult:@var{m1} (sign_extend:@var{m1} (ashiftrt:@var{m2} @var{x} @var{s}))
+(sign_extend:@var{m1} @var{y}))}, and likewise for patterns using @code{zero_extend}
+and @code{lshiftrt}.  If the second operand of @code{mult} is also a shift,
+then that is extended also.
+
 @end itemize
 
 Further canonicalization rules are defined in the function
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -1000,6 +1000,34 @@ simplify_unary_operation_1 (enum rtx_code code, enum machine_mode mode, rtx op)
 	  && GET_CODE (XEXP (XEXP (op, 0), 1)) == LABEL_REF)
 	return XEXP (op, 0);
 
+      /* Extending a widening multiplication should be canonicalized to
+	 a wider widening multiplication.  */
+      if (GET_CODE (op) == MULT)
+	{
+	  rtx lhs = XEXP (op, 0);
+	  rtx rhs = XEXP (op, 1);
+	  enum rtx_code lcode = GET_CODE (lhs);
+	  enum rtx_code rcode = GET_CODE (rhs);
+
+	  /* Widening multiplies usually extend both operands, but sometimes
+	     they use a shift to extract a portion of a register. We assume
+	     it is safe to widen all such operands because other examples
+	     won't match real instructions.  */
+	  if ((lcode == SIGN_EXTEND || lcode == ASHIFTRT)
+	      && (rcode == SIGN_EXTEND || rcode == ASHIFTRT))
+	    {
+	      enum machine_mode lmode = GET_MODE (lhs);
+	      enum machine_mode rmode = GET_MODE (lhs);
+	      return simplify_gen_binary (MULT, mode,
+					  simplify_gen_unary (SIGN_EXTEND,
+							      mode,
+							      lhs, lmode),
+					  simplify_gen_unary (SIGN_EXTEND,
+							      mode,
+							      rhs, rmode));
+	    }
+	}
+
       /* Check for a sign extension of a subreg of a promoted
 	 variable, where the promotion is sign-extended, and the
 	 target mode is the same as the variable's promotion.  */
@@ -1071,6 +1099,34 @@ simplify_unary_operation_1 (enum rtx_code code, enum machine_mode mode, rtx op)
 	  && GET_MODE_SIZE (mode) <= GET_MODE_SIZE (GET_MODE (XEXP (op, 0))))
 	return rtl_hooks.gen_lowpart_no_emit (mode, op);
 
+      /* Extending a widening multiplication should be canonicalized to
+	 a wider widening multiplication.  */
+      if (GET_CODE (op) == MULT)
+	{
+	  rtx lhs = XEXP (op, 0);
+	  rtx rhs = XEXP (op, 1);
+	  enum rtx_code lcode = GET_CODE (lhs);
+	  enum rtx_code rcode = GET_CODE (rhs);
+
+	  /* Widening multiplies usually extend both operands, but sometimes
+	     they use a shift to extract a portion of a register. We assume
+	     it is safe to widen all such operands because other examples
+	     won't match real instructions.  */
+	  if ((lcode == ZERO_EXTEND || lcode == LSHIFTRT)
+	      && (rcode == ZERO_EXTEND || rcode == LSHIFTRT))
+	    {
+	      enum machine_mode lmode = GET_MODE (lhs);
+	      enum machine_mode rmode = GET_MODE (lhs);
+	      return simplify_gen_binary (MULT, mode,
+					  simplify_gen_unary (ZERO_EXTEND,
+							      mode,
+							      lhs, lmode),
+					  simplify_gen_unary (ZERO_EXTEND,
+							      mode,
+							      rhs, rmode));
+	    }
+	}
+
       /* (zero_extend:M (zero_extend:N <X>)) is (zero_extend:M <X>).  */
       if (GET_CODE (op) == ZERO_EXTEND)
 	return simplify_gen_unary (ZERO_EXTEND, mode, XEXP (op, 0),
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/mla-2.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=armv7-a" } */
+
+long long foolong (long long x, short *a, short *b)
+{
+    return x + *a * *b;
+}
+
+/* { dg-final { scan-assembler "smlalbb" } } */