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* [gcc r12-3075] Improved handling of division/modulus in bit CCP.
@ 2021-08-23 11:45 Roger Sayle
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From: Roger Sayle @ 2021-08-23 11:45 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:e7721590e08e6d87adc879f8d549557cbe2bb7bb
commit r12-3075-ge7721590e08e6d87adc879f8d549557cbe2bb7bb
Author: Roger Sayle <roger@nextmovesoftware.com>
Date: Mon Aug 23 12:44:37 2021 +0100
Improved handling of division/modulus in bit CCP.
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.
Diff:
---
gcc/tree-ssa-ccp.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 62 insertions(+)
diff --git a/gcc/tree-ssa-ccp.c b/gcc/tree-ssa-ccp.c
index 1a63ae5f104..1a94aebb0e4 100644
--- a/gcc/tree-ssa-ccp.c
+++ b/gcc/tree-ssa-ccp.c
@@ -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:;
}
}
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