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Improved handling of shifts/rotates in bit CCP.

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This patch is the next in the series to improve bit bounds in tree-ssa's
bit CCP pass, this time: bounds for shifts and rotates by unknown amounts.
This allows us to optimize expressions such as ((x&15)<<(y&24))&64.
In this case, the expression (y&24) contains only two unknown bits,
and can therefore have only four possible values: 0, 8, 16 and 24.
From this (x&15)<<(y&24) has the nonzero bits 0x0f0f0f0f, and from
that ((x&15)<<(y&24))&64 must always be zero.

One clever use of computer science in this patch is the use of XOR
to efficiently enumerate bit patterns in Gray code order.  As the
order in which we generate values is not significant, it's faster
and more convenient to enumerate values by flipping one bit at a
time, rather than in numerical order [which would require carry
bits and additional logic].

There's a pre-existing ??? comment in tree-ssa-ccp.c that we should
eventually be able to optimize (x<<(y|8))&255, but this patch takes the
conservatively paranoid approach of only optimizing cases where the
shift/rotate is guaranteed to be less than the target precision, and
therefore avoids changing any cases that potentially might invoke
undefined behavior.  This patch does optimize (x<<((y&31)|8))&255.

2021-08-26  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* tree-ssa-ccp.c (get_individual_bits): Helper function to
	extract the individual bits from a widest_int constant (mask).
	(gray_code_bit_flips): New read-only table for effiently
	enumerating permutations/combinations of bits.
	(bit_value_binop) [LROTATE_EXPR, RROTATE_EXPR]: Handle rotates
	by unknown counts that are guaranteed less than the target
	precision and four or fewer unknown bits by enumeration.
	[LSHIFT_EXPR, RSHIFT_EXPR]: Likewise, also handle shifts by
	enumeration under the same conditions.  Handle remaining
	shifts as a mask based upon the minimum possible shift value.

gcc/testsuite/ChangeLog
	* gcc.dg/tree-ssa/ssa-ccp-41.c: New test case.
This commit is contained in:
Roger Sayle 2021-08-26 18:57:00 +01:00
parent a2d9b55829
commit b2ef23239f
2 changed files with 171 additions and 0 deletions
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11
gcc/testsuite/gcc.dg/tree-ssa/ssa-ccp-41.c Normal file
View File

@ -0,0 +1,11 @@
/* { dg-do compile } */
/* { dg-options "-O2 -fdump-tree-optimized" } */
int foo(int x)
{
int p = x & 24;
int r = 1 << p;
return r & (1<<17);
}
/* { dg-final { scan-tree-dump "return 0;" "optimized" } } */

160
gcc/tree-ssa-ccp.c
View File

@ -1448,6 +1448,34 @@ bit_value_mult_const (signop sgn, int width,
*mask = wi::ext (sum_mask, width, sgn);
}
/* Fill up to MAX values in the BITS array with values representing
each of the non-zero bits in the value X. Returns the number of
bits in X (capped at the maximum value MAX). For example, an X
value 11, places 1, 2 and 8 in BITS and returns the value 3. */
unsigned int
get_individual_bits (widest_int *bits, widest_int x, unsigned int max)
{
unsigned int count = 0;
while (count < max && x != 0)
{
int bitpos = wi::ctz (x);
bits[count] = wi::lshift (1, bitpos);
x ^= bits[count];
count++;
}
return count;
}
/* Array of 2^N - 1 values representing the bits flipped between
consecutive Gray codes. This is used to efficiently enumerate
all permutations on N bits using XOR. */
static const unsigned char gray_code_bit_flips[63] = {
0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4,
0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 5,
0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4,
0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
};
/* Apply the operation CODE in type TYPE to the value, mask pairs
R1VAL, R1MASK and R2VAL, R2MASK representing a values of type R1TYPE
@ -1525,6 +1553,48 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
}
}
}
else if (wi::ltu_p (r2val | r2mask, width)
&& wi::popcount (r2mask) <= 4)
{
widest_int bits[4];
widest_int res_val, res_mask;
widest_int tmp_val, tmp_mask;
widest_int shift = wi::bit_and_not (r2val, r2mask);
unsigned int bit_count = get_individual_bits (bits, r2mask, 4);
unsigned int count = (1 << bit_count) - 1;
/* Initialize result to rotate by smallest value of shift. */
if (code == RROTATE_EXPR)
{
res_mask = wi::rrotate (r1mask, shift, width);
res_val = wi::rrotate (r1val, shift, width);
}
else
{
res_mask = wi::lrotate (r1mask, shift, width);
res_val = wi::lrotate (r1val, shift, width);
}
/* Iterate through the remaining values of shift. */
for (unsigned int i=0; i<count; i++)
{
shift ^= bits[gray_code_bit_flips[i]];
if (code == RROTATE_EXPR)
{
tmp_mask = wi::rrotate (r1mask, shift, width);
tmp_val = wi::rrotate (r1val, shift, width);
}
else
{
tmp_mask = wi::lrotate (r1mask, shift, width);
tmp_val = wi::lrotate (r1val, shift, width);
}
/* Accumulate the result. */
res_mask |= tmp_mask | (res_val ^ tmp_val);
}
*val = wi::bit_and_not (res_val, res_mask);
*mask = res_mask;
}
break;
case LSHIFT_EXPR:
@ -1556,6 +1626,96 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
}
}
}
else if (wi::ltu_p (r2val | r2mask, width))
{
if (wi::popcount (r2mask) <= 4)
{
widest_int bits[4];
widest_int arg_val, arg_mask;
widest_int res_val, res_mask;
widest_int tmp_val, tmp_mask;
widest_int shift = wi::bit_and_not (r2val, r2mask);
unsigned int bit_count = get_individual_bits (bits, r2mask, 4);
unsigned int count = (1 << bit_count) - 1;
/* Initialize result to shift by smallest value of shift. */
if (code == RSHIFT_EXPR)
{
arg_mask = wi::ext (r1mask, width, sgn);
arg_val = wi::ext (r1val, width, sgn);
res_mask = wi::rshift (arg_mask, shift, sgn);
res_val = wi::rshift (arg_val, shift, sgn);
}
else
{
arg_mask = r1mask;
arg_val = r1val;
res_mask = arg_mask << shift;
res_val = arg_val << shift;
}
/* Iterate through the remaining values of shift. */
for (unsigned int i=0; i<count; i++)
{
shift ^= bits[gray_code_bit_flips[i]];
if (code == RSHIFT_EXPR)
{
tmp_mask = wi::rshift (arg_mask, shift, sgn);
tmp_val = wi::rshift (arg_val, shift, sgn);
}
else
{
tmp_mask = arg_mask << shift;
tmp_val = arg_val << shift;
}
/* Accumulate the result. */
res_mask |= tmp_mask | (res_val ^ tmp_val);
}
res_mask = wi::ext (res_mask, width, sgn);
res_val = wi::ext (res_val, width, sgn);
*val = wi::bit_and_not (res_val, res_mask);
*mask = res_mask;
}
else if ((r1val | r1mask) == 0)
{
/* Handle shifts of zero to avoid undefined wi::ctz below. */
*mask = 0;
*val = 0;
}
else if (code == LSHIFT_EXPR)
{
widest_int tmp = wi::mask <widest_int> (width, false);
tmp <<= wi::ctz (r1val | r1mask);
tmp <<= wi::bit_and_not (r2val, r2mask);
*mask = wi::ext (tmp, width, sgn);
*val = 0;
}
else if (!wi::neg_p (r1val | r1mask, sgn))
{
/* Logical right shift, or zero sign bit. */
widest_int arg = r1val | r1mask;
int lzcount = wi::clz (arg);
lzcount -= wi::get_precision (arg) - width;
widest_int tmp = wi::mask <widest_int> (width, false);
tmp = wi::lrshift (tmp, lzcount);
tmp = wi::lrshift (tmp, wi::bit_and_not (r2val, r2mask));
*mask = wi::ext (tmp, width, sgn);
*val = 0;
}
else if (!wi::neg_p (r1mask))
{
/* Arithmetic right shift with set sign bit. */
widest_int arg = wi::bit_and_not (r1val, r1mask);
int sbcount = wi::clrsb (arg);
sbcount -= wi::get_precision (arg) - width;
widest_int tmp = wi::mask <widest_int> (width, false);
tmp = wi::lrshift (tmp, sbcount);
tmp = wi::lrshift (tmp, wi::bit_and_not (r2val, r2mask));
*mask = wi::sext (tmp, width);
tmp = wi::bit_not (tmp);
*val = wi::sext (tmp, width);
}
}
break;
case PLUS_EXPR: