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tcg/optimize: Propagate sign info for shifting 

For constant shifts, we can simply shift the s_mask.

For variable shifts, we know that sar does not reduce
the s_mask, which helps for sequences like

    ext32s_i64  t, in
    sar_i64     t, t, v
    ext32s_i64  out, t

allowing the final extend to be eliminated.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2021-08-26 13:24:59 -07:00
parent 2b9d0c59ed
commit 93a967fbb5
1 changed files with 47 additions and 3 deletions
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50
tcg/optimize.c
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@ -85,6 +85,18 @@ static uint64_t smask_from_zmask(uint64_t zmask)
return ~(~0ull >> rep);
}
/*
* Recreate a properly left-aligned smask after manipulation.
* Some bit-shuffling, particularly shifts and rotates, may
* retain sign bits on the left, but may scatter disconnected
* sign bits on the right. Retain only what remains to the left.
*/
static uint64_t smask_from_smask(int64_t smask)
{
/* Only the 1 bits are significant for smask */
return smask_from_zmask(~smask);
}
static inline TempOptInfo *ts_info(TCGTemp *ts)
{
return ts->state_ptr;
@ -1843,18 +1855,50 @@ static bool fold_sextract(OptContext *ctx, TCGOp *op)
static bool fold_shift(OptContext *ctx, TCGOp *op)
{
uint64_t s_mask, z_mask, sign;
if (fold_const2(ctx, op) ||
fold_ix_to_i(ctx, op, 0) ||
fold_xi_to_x(ctx, op, 0)) {
return true;
}
s_mask = arg_info(op->args[1])->s_mask;
z_mask = arg_info(op->args[1])->z_mask;
if (arg_is_const(op->args[2])) {
ctx->z_mask = do_constant_folding(op->opc, ctx->type,
arg_info(op->args[1])->z_mask,
arg_info(op->args[2])->val);
int sh = arg_info(op->args[2])->val;
ctx->z_mask = do_constant_folding(op->opc, ctx->type, z_mask, sh);
s_mask = do_constant_folding(op->opc, ctx->type, s_mask, sh);
ctx->s_mask = smask_from_smask(s_mask);
return fold_masks(ctx, op);
}
switch (op->opc) {
CASE_OP_32_64(sar):
/*
* Arithmetic right shift will not reduce the number of
* input sign repetitions.
*/
ctx->s_mask = s_mask;
break;
CASE_OP_32_64(shr):
/*
* If the sign bit is known zero, then logical right shift
* will not reduced the number of input sign repetitions.
*/
sign = (s_mask & -s_mask) >> 1;
if (!(z_mask & sign)) {
ctx->s_mask = s_mask;
}
break;
default:
break;
}
return false;
}