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Fold (X<<C1)^(X<<C2) to a multiplication when possible.

Browse source 
The easiest way to motivate these additions to match.pd is with the
following example:

unsigned int foo(unsigned char i) {
  return i | (i<<8) | (i<<16) | (i<<24);
}

which mainline with -O2 on x86_64 currently generates:
foo:	movzbl  %dil, %edi
	movl    %edi, %eax
	movl    %edi, %edx
	sall    $8, %eax
	sall    $16, %edx
	orl     %edx, %eax
	orl     %edi, %eax
	sall    $24, %edi
	orl     %edi, %eax
	ret

but with this patch now becomes:
foo:	movzbl  %dil, %eax
        imull   $16843009, %eax, %eax
        ret

Interestingly, this transformation is already applied when using
addition, allowing synth_mult to select an optimal sequence, but
not when using the equivalent bit-wise ior or xor operators.

The solution is to use tree_nonzero_bits to check that the
potentially non-zero bits of each operand don't overlap, which
ensures that BIT_IOR_EXPR and BIT_XOR_EXPR produce the same
results as PLUS_EXPR, which effectively generalizes the old
fold_plusminus_mult_expr.  Technically, the transformation
is to canonicalize (X*C1)|(X*C2) and (X*C1)^(X*C2) to
X*(C1+C2) where X and X<<C are considered special cases.

2021-08-04  Roger Sayle  <roger@nextmovesoftware.com>
	    Marc Glisse  <marc.glisse@inria.fr>

gcc/ChangeLog
	* match.pd (bit_ior, bit_xor): Canonicalize (X*C1)|(X*C2) and
	(X*C1)^(X*C2) as X*(C1+C2), and related variants, using
	tree_nonzero_bits to ensure that operands are bit-wise disjoint.

gcc/testsuite/ChangeLog
	* gcc.dg/fold-ior-4.c: New test.
This commit is contained in:
Roger Sayle 2021-08-04 14:19:14 +01:00
parent 0d04fe4923
commit 96146e61cd
2 changed files with 117 additions and 0 deletions
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56
gcc/match.pd
View file

@ -2833,6 +2833,62 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(convert (mult (convert:t @0) { cst; })))))
#endif
/* Canonicalize (X*C1)|(X*C2) and (X*C1)^(X*C2) to (C1+C2)*X when
tree_nonzero_bits allows IOR and XOR to be treated like PLUS.
Likewise, handle (X<<C3) and X as legitimate variants of X*C. */
(for op (bit_ior bit_xor)
(simplify
(op (mult:s@0 @1 INTEGER_CST@2)
(mult:s@3 @1 INTEGER_CST@4))
(if (INTEGRAL_TYPE_P (type) && TYPE_OVERFLOW_WRAPS (type)
&& (tree_nonzero_bits (@0) & tree_nonzero_bits (@3)) == 0)
(mult @1
{ wide_int_to_tree (type, wi::to_wide (@2) + wi::to_wide (@4)); })))
(simplify
(op:c (mult:s@0 @1 INTEGER_CST@2)
(lshift:s@3 @1 INTEGER_CST@4))
(if (INTEGRAL_TYPE_P (type) && TYPE_OVERFLOW_WRAPS (type)
&& tree_int_cst_sgn (@4) > 0
&& (tree_nonzero_bits (@0) & tree_nonzero_bits (@3)) == 0)
(with { wide_int wone = wi::one (TYPE_PRECISION (type));
wide_int c = wi::add (wi::to_wide (@2),
wi::lshift (wone, wi::to_wide (@4))); }
(mult @1 { wide_int_to_tree (type, c); }))))
(simplify
(op:c (mult:s@0 @1 INTEGER_CST@2)
@1)
(if (INTEGRAL_TYPE_P (type) && TYPE_OVERFLOW_WRAPS (type)
&& (tree_nonzero_bits (@0) & tree_nonzero_bits (@1)) == 0)
(mult @1
{ wide_int_to_tree (type,
wi::add (wi::to_wide (@2), 1)); })))
(simplify
(op (lshift:s@0 @1 INTEGER_CST@2)
(lshift:s@3 @1 INTEGER_CST@4))
(if (INTEGRAL_TYPE_P (type)
&& tree_int_cst_sgn (@2) > 0
&& tree_int_cst_sgn (@4) > 0
&& (tree_nonzero_bits (@0) & tree_nonzero_bits (@3)) == 0)
(with { tree t = type;
if (!TYPE_OVERFLOW_WRAPS (t))
t = unsigned_type_for (t);
wide_int wone = wi::one (TYPE_PRECISION (t));
wide_int c = wi::add (wi::lshift (wone, wi::to_wide (@2)),
wi::lshift (wone, wi::to_wide (@4))); }
(convert (mult:t (convert:t @1) { wide_int_to_tree (t,c); })))))
(simplify
(op:c (lshift:s@0 @1 INTEGER_CST@2)
@1)
(if (INTEGRAL_TYPE_P (type)
&& tree_int_cst_sgn (@2) > 0
&& (tree_nonzero_bits (@0) & tree_nonzero_bits (@1)) == 0)
(with { tree t = type;
if (!TYPE_OVERFLOW_WRAPS (t))
t = unsigned_type_for (t);
wide_int wone = wi::one (TYPE_PRECISION (t));
wide_int c = wi::add (wi::lshift (wone, wi::to_wide (@2)), wone); }
(convert (mult:t (convert:t @1) { wide_int_to_tree (t, c); }))))))
/* Simplifications of MIN_EXPR, MAX_EXPR, fmin() and fmax(). */
(for minmax (min max FMIN_ALL FMAX_ALL)

61
gcc/testsuite/gcc.dg/fold-ior-4.c Normal file
View file

@ -0,0 +1,61 @@
/* { dg-do compile } */
/* { dg-options "-O2 -fdump-tree-optimized" } */
unsigned int test_ior(unsigned char i)
{
return i | (i<<8) | (i<<16) | (i<<24);
}
unsigned int test_xor(unsigned char i)
{
return i ^ (i<<8) ^ (i<<16) ^ (i<<24);
}
unsigned int test_ior_1s(unsigned char i)
{
return i | (i<<8);
}
unsigned int test_ior_1u(unsigned char i)
{
unsigned int t = i;
return t | (t<<8);
}
unsigned int test_xor_1s(unsigned char i)
{
return i ^ (i<<8);
}
unsigned int test_xor_1u(unsigned char i)
{
unsigned int t = i;
return t ^ (t<<8);
}
unsigned int test_ior_2s(unsigned char i)
{
return (i<<8) | (i<<16);
}
unsigned int test_ior_2u(unsigned char i)
{
unsigned int t = i;
return (t<<8) | (t<<16);
}
unsigned int test_xor_2s(unsigned char i)
{
return (i<<8) ^ (i<<16);
}
unsigned int test_xor_2u(unsigned char i)
{
unsigned int t = i;
return (t<<8) ^ (t<<16);
}
/* { dg-final { scan-tree-dump-not " \\^ " "optimized" } } */
/* { dg-final { scan-tree-dump-not " \\| " "optimized" } } */
/* { dg-final { scan-tree-dump-times " \\* 16843009" 2 "optimized" } } */