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target/arm: Implement increased precision FRECPE

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Implement the increased precision variation of FRECPE.  In the
pseudocode this corresponds to the handling of the
"increasedprecision" boolean in the FPRecipEstimate() and
RecipEstimate() functions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Peter Maydell 2025-02-01 16:39:57 +00:00
parent 0ff5c021f0
commit c1567205e0
1 changed files with 46 additions and 8 deletions
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54
target/arm/vfp_helper.c
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@ -733,6 +733,33 @@ static int recip_estimate(int input)
return r;
}
/*
* Increased precision version:
* input is a 13 bit fixed point number
* input range 2048 .. 4095 for a number from 0.5 <= x < 1.0.
* result range 4096 .. 8191 for a number from 1.0 to 2.0
*/
static int recip_estimate_incprec(int input)
{
int a, b, r;
assert(2048 <= input && input < 4096);
a = (input * 2) + 1;
/*
* The pseudocode expresses this as an operation on infinite
* precision reals where it calculates 2^25 / a and then looks
* at the error between that and the rounded-down-to-integer
* value to see if it should instead round up. We instead
* follow the same approach as the pseudocode for the 8-bit
* precision version, and calculate (2 * (2^25 / a)) as an
* integer so we can do the "add one and halve" to round it.
* So the 1 << 26 here is correct.
*/
b = (1 << 26) / a;
r = (b + 1) >> 1;
assert(4096 <= r && r < 8192);
return r;
}
/*
* Common wrapper to call recip_estimate
*
@ -742,7 +769,8 @@ static int recip_estimate(int input)
* callee.
*/
static uint64_t call_recip_estimate(int *exp, int exp_off, uint64_t frac)
static uint64_t call_recip_estimate(int *exp, int exp_off, uint64_t frac,
bool increasedprecision)
{
uint32_t scaled, estimate;
uint64_t result_frac;
@ -758,12 +786,22 @@ static uint64_t call_recip_estimate(int *exp, int exp_off, uint64_t frac)
}
}
/* scaled = UInt('1':fraction<51:44>) */
scaled = deposit32(1 << 8, 0, 8, extract64(frac, 44, 8));
estimate = recip_estimate(scaled);
if (increasedprecision) {
/* scaled = UInt('1':fraction<51:41>) */
scaled = deposit32(1 << 11, 0, 11, extract64(frac, 41, 11));
estimate = recip_estimate_incprec(scaled);
} else {
/* scaled = UInt('1':fraction<51:44>) */
scaled = deposit32(1 << 8, 0, 8, extract64(frac, 44, 8));
estimate = recip_estimate(scaled);
}
result_exp = exp_off - *exp;
result_frac = deposit64(0, 44, 8, estimate);
if (increasedprecision) {
result_frac = deposit64(0, 40, 12, estimate);
} else {
result_frac = deposit64(0, 44, 8, estimate);
}
if (result_exp == 0) {
result_frac = deposit64(result_frac >> 1, 51, 1, 1);
} else if (result_exp == -1) {
@ -832,7 +870,7 @@ uint32_t HELPER(recpe_f16)(uint32_t input, float_status *fpst)
}
f64_frac = call_recip_estimate(&f16_exp, 29,
((uint64_t) f16_frac) << (52 - 10));
((uint64_t) f16_frac) << (52 - 10), false);
/* result = sign : result_exp<4:0> : fraction<51:42> */
f16_val = deposit32(0, 15, 1, f16_sign);
@ -885,7 +923,7 @@ static float32 do_recpe_f32(float32 input, float_status *fpst, bool rpres)
}
f64_frac = call_recip_estimate(&f32_exp, 253,
((uint64_t) f32_frac) << (52 - 23));
((uint64_t) f32_frac) << (52 - 23), rpres);
/* result = sign : result_exp<7:0> : fraction<51:29> */
f32_val = deposit32(0, 31, 1, f32_sign);
@ -943,7 +981,7 @@ float64 HELPER(recpe_f64)(float64 input, float_status *fpst)
return float64_set_sign(float64_zero, float64_is_neg(f64));
}
f64_frac = call_recip_estimate(&f64_exp, 2045, f64_frac);
f64_frac = call_recip_estimate(&f64_exp, 2045, f64_frac, false);
/* result = sign : result_exp<10:0> : fraction<51:0>; */
f64_val = deposit64(0, 63, 1, f64_sign);