From 408579c9c9b8fee20e1d8114489ce2b93872767c Mon Sep 17 00:00:00 2001 From: Roger Sayle Date: Tue, 17 Aug 2021 14:50:54 +0100 Subject: Improved handling of MULT_EXPR in bit CCP. This patch allows GCC to constant fold (i | (i<<16)) | ((i<<24) | (i<<8)), where i is an unsigned char, or the equivalent (i*65537) | (i*16777472), to i*16843009. The trick is to teach tree_nonzero_bits which bits may be set in the result of a multiplication by a constant given which bits are potentially set in the operands. This allows the optimizations recently added to match.pd to catch more cases. The required mask/value pair from a multiplication may be calculated using a classical shift-and-add algorithm, given we already have implementations for both addition and shift by constant. To keep this optimization "cheap", this functionality is only used if the constant multiplier has a few bits set (unless flag_expensive_optimizations), and we provide a special case fast-path implementation for the common case where the (non-constant) operand has no bits that are guaranteed to be set. I have no evidence that this functionality causes performance issues, it's just that sparse multipliers provide the largest benefit to CCP. 2021-08-17 Roger Sayle gcc/ChangeLog * tree-ssa-ccp.c (bit_value_mult_const): New helper function to calculate the mask-value pair result of a multiplication by an unsigned constant. (bit_value_binop) [MULT_EXPR]: Call it from here for multiplications by (sparse) non-negative constants. gcc/testsuite/ChangeLog * gcc.dg/fold-ior-5.c: New test case. --- gcc/tree-ssa-ccp.c | 105 ++++++++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 87 insertions(+), 18 deletions(-) (limited to 'gcc/tree-ssa-ccp.c') diff --git a/gcc/tree-ssa-ccp.c b/gcc/tree-ssa-ccp.c index 003c9c2..8e4d8ae 100644 --- a/gcc/tree-ssa-ccp.c +++ b/gcc/tree-ssa-ccp.c @@ -1389,6 +1389,66 @@ bit_value_unop (enum tree_code code, signop type_sgn, int type_precision, } } +/* Determine the mask pair *VAL and *MASK from multiplying the + argument mask pair RVAL, RMASK by the unsigned constant C. */ +void +bit_value_mult_const (signop sgn, int width, + widest_int *val, widest_int *mask, + const widest_int &rval, const widest_int &rmask, + widest_int c) +{ + widest_int sum_mask = 0; + + /* Ensure rval_lo only contains known bits. */ + widest_int rval_lo = wi::bit_and_not (rval, rmask); + + if (rval_lo != 0) + { + /* General case (some bits of multiplicand are known set). */ + widest_int sum_val = 0; + while (c != 0) + { + /* Determine the lowest bit set in the multiplier. */ + int bitpos = wi::ctz (c); + widest_int term_mask = rmask << bitpos; + widest_int term_val = rval_lo << bitpos; + + /* sum += term. */ + widest_int lo = sum_val + term_val; + widest_int hi = (sum_val | sum_mask) + (term_val | term_mask); + sum_mask |= term_mask | (lo ^ hi); + sum_val = lo; + + /* Clear this bit in the multiplier. */ + c ^= wi::lshift (1, bitpos); + } + /* Correctly extend the result value. */ + *val = wi::ext (sum_val, width, sgn); + } + else + { + /* Special case (no bits of multiplicand are known set). */ + while (c != 0) + { + /* Determine the lowest bit set in the multiplier. */ + int bitpos = wi::ctz (c); + widest_int term_mask = rmask << bitpos; + + /* sum += term. */ + widest_int hi = sum_mask + term_mask; + sum_mask |= term_mask | hi; + + /* Clear this bit in the multiplier. */ + c ^= wi::lshift (1, bitpos); + } + *val = 0; + } + + /* Correctly extend the result mask. */ + *mask = wi::ext (sum_mask, width, sgn); +} + + /* Apply the operation CODE in type TYPE to the value, mask pairs R1VAL, R1MASK and R2VAL, R2MASK representing a values of type R1TYPE and R2TYPE and set the value, mask pair *VAL and *MASK to the result. */ @@ -1533,24 +1593,33 @@ bit_value_binop (enum tree_code code, signop sgn, int width, } case MULT_EXPR: - { - /* Just track trailing zeros in both operands and transfer - them to the other. */ - int r1tz = wi::ctz (r1val | r1mask); - int r2tz = wi::ctz (r2val | r2mask); - if (r1tz + r2tz >= width) - { - *mask = 0; - *val = 0; - } - else if (r1tz + r2tz > 0) - { - *mask = wi::ext (wi::mask (r1tz + r2tz, true), - width, sgn); - *val = 0; - } - break; - } + if (r2mask == 0 + && !wi::neg_p (r2val, sgn) + && (flag_expensive_optimizations || wi::popcount (r2val) < 8)) + bit_value_mult_const (sgn, width, val, mask, r1val, r1mask, r2val); + else if (r1mask == 0 + && !wi::neg_p (r1val, sgn) + && (flag_expensive_optimizations || wi::popcount (r1val) < 8)) + bit_value_mult_const (sgn, width, val, mask, r2val, r2mask, r1val); + else + { + /* Just track trailing zeros in both operands and transfer + them to the other. */ + int r1tz = wi::ctz (r1val | r1mask); + int r2tz = wi::ctz (r2val | r2mask); + if (r1tz + r2tz >= width) + { + *mask = 0; + *val = 0; + } + else if (r1tz + r2tz > 0) + { + *mask = wi::ext (wi::mask (r1tz + r2tz, true), + width, sgn); + *val = 0; + } + } + break; case EQ_EXPR: case NE_EXPR: -- cgit v1.1