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Improved handling of division/modulus in bit CCP.

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This patch implements support for TRUNC_MOD_EXPR and TRUNC_DIV_EXPR
in tree-ssa's bit CCP pass.  This is mostly for completeness, as the
VRP pass already provides better bounds for these operations, but
seeing mask values of all_ones in my debugging/instrumentation logs
seemed overly pessimistic.  With this patch, the expression X%10
has a nonzero bits of 0x0f (for unsigned X), likewise (X&1)/3 has
a known value of zero, and (X&3)/3 has a nonzero bits mask of 0x1.

2021-08-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* tree-ssa-ccp.c (bit_value_binop) [TRUNC_MOD_EXPR, TRUNC_DIV_EXPR]:
	Provide bounds for unsigned (and signed with non-negative operands)
	division and modulus.
This commit is contained in:
Roger Sayle 2021-08-23 12:44:37 +01:00
parent 7e5f9ead16
commit e7721590e0
1 changed files with 62 additions and 0 deletions
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62
gcc/tree-ssa-ccp.c
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@ -1736,6 +1736,68 @@ bit_value_binop (enum tree_code code, signop sgn, int width,
break;
}
case TRUNC_MOD_EXPR:
{
widest_int r1max = r1val | r1mask;
widest_int r2max = r2val | r2mask;
if (sgn == UNSIGNED
|| (!wi::neg_p (r1max) && !wi::neg_p (r2max)))
{
/* Confirm R2 has some bits set, to avoid division by zero. */
widest_int r2min = wi::bit_and_not (r2val, r2mask);
if (r2min != 0)
{
/* R1 % R2 is R1 if R1 is always less than R2. */
if (wi::ltu_p (r1max, r2min))
{
*mask = r1mask;
*val = r1val;
}
else
{
/* R1 % R2 is always less than the maximum of R2. */
unsigned int lzcount = wi::clz (r2max);
unsigned int bits = wi::get_precision (r2max) - lzcount;
if (r2max == wi::lshift (1, bits))
bits--;
*mask = wi::mask <widest_int> (bits, false);
*val = 0;
}
}
}
}
break;
case TRUNC_DIV_EXPR:
{
widest_int r1max = r1val | r1mask;
widest_int r2max = r2val | r2mask;
if (sgn == UNSIGNED
|| (!wi::neg_p (r1max) && !wi::neg_p (r2max)))
{
/* Confirm R2 has some bits set, to avoid division by zero. */
widest_int r2min = wi::bit_and_not (r2val, r2mask);
if (r2min != 0)
{
/* R1 / R2 is zero if R1 is always less than R2. */
if (wi::ltu_p (r1max, r2min))
{
*mask = 0;
*val = 0;
}
else
{
widest_int upper = wi::udiv_trunc (r1max, r2min);
unsigned int lzcount = wi::clz (upper);
unsigned int bits = wi::get_precision (upper) - lzcount;
*mask = wi::mask <widest_int> (bits, false);
*val = 0;
}
}
}
}
break;
default:;
}
}