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fpu: Handle m68k extended precision denormals properly

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Motorola treats denormals with explicit integer bit set as
having unbiased exponent 0, unlike Intel which treats it as
having unbiased exponent 1 (more like all other IEEE formats
that have no explicit integer bit).

Add a flag on FloatFmt to differentiate the behaviour.

Reported-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2023-08-20 17:28:33 -07:00
parent 00f9ef8f3d
commit 722460652b
4 changed files with 66 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
fpu/softfloat-parts.c.inc
View File

@ -118,7 +118,8 @@ static void partsN(canonicalize)(FloatPartsN *p, float_status *status,
} else {
int shift = frac_normalize(p);
p->cls = float_class_normal;
p->exp = fmt->frac_shift - fmt->exp_bias - shift + 1;
p->exp = fmt->frac_shift - fmt->exp_bias
- shift + !fmt->m68k_denormal;
}
} else if (likely(p->exp < fmt->exp_max) || fmt->arm_althp) {
p->cls = float_class_normal;
@ -256,7 +257,7 @@ static void partsN(uncanon_normal)(FloatPartsN *p, float_status *s,
is_tiny = !frac_addi(&discard, p, inc);
}
frac_shrjam(p, 1 - exp);
frac_shrjam(p, !fmt->m68k_denormal - exp);
if (p->frac_lo & round_mask) {
/* Need to recompute round-to-even/round-to-odd. */
@ -287,7 +288,7 @@ static void partsN(uncanon_normal)(FloatPartsN *p, float_status *s,
p->frac_lo &= ~round_mask;
}
exp = (p->frac_hi & DECOMPOSED_IMPLICIT_BIT) != 0;
exp = (p->frac_hi & DECOMPOSED_IMPLICIT_BIT) && !fmt->m68k_denormal;
frac_shr(p, frac_shift);
if (is_tiny && (flags & float_flag_inexact)) {

9
fpu/softfloat.c
View File

@ -517,6 +517,7 @@ typedef struct {
* round_mask: bits below lsb which must be rounded
* The following optional modifiers are available:
* arm_althp: handle ARM Alternative Half Precision
* m68k_denormal: explicit integer bit for extended precision may be 1
*/
typedef struct {
int exp_size;
@ -526,6 +527,7 @@ typedef struct {
int frac_size;
int frac_shift;
bool arm_althp;
bool m68k_denormal;
uint64_t round_mask;
} FloatFmt;
@ -576,7 +578,12 @@ static const FloatFmt float128_params = {
static const FloatFmt floatx80_params[3] = {
[floatx80_precision_s] = { FLOATX80_PARAMS(23) },
[floatx80_precision_d] = { FLOATX80_PARAMS(52) },
[floatx80_precision_x] = { FLOATX80_PARAMS(64) },
[floatx80_precision_x] = {
FLOATX80_PARAMS(64),
#ifdef TARGET_M68K
.m68k_denormal = true,
#endif
},
};
/* Unpack a float to parts, but do not canonicalize. */

2
tests/tcg/m68k/Makefile.target
View File

@ -4,7 +4,7 @@
#
VPATH += $(SRC_PATH)/tests/tcg/m68k
TESTS += trap
TESTS += trap denormal
# On m68k Linux supports 4k and 8k pages (but 8k is currently broken)
EXTRA_RUNS+=run-test-mmap-4096 # run-test-mmap-8192

53
tests/tcg/m68k/denormal.c Normal file
View File

@ -0,0 +1,53 @@
/*
* Test m68k extended double denormals.
*/
#include <stdio.h>
#include <stdint.h>
#define TEST(X, Y) { X, Y, X * Y }
static volatile long double test[][3] = {
TEST(0x1p+16383l, 0x1p-16446l),
TEST(0x1.1p-8223l, 0x1.1p-8224l),
TEST(1.0l, 0x1p-16383l),
};
#undef TEST
static void dump_ld(const char *label, long double ld)
{
union {
long double d;
struct {
uint32_t exp:16;
uint32_t space:16;
uint32_t h;
uint32_t l;
};
} u;
u.d = ld;
printf("%12s: % -27La 0x%04x 0x%08x 0x%08x\n", label, u.d, u.exp, u.h, u.l);
}
int main(void)
{
int i, n = sizeof(test) / sizeof(test[0]), err = 0;
for (i = 0; i < n; ++i) {
long double x = test[i][0];
long double y = test[i][1];
long double build_mul = test[i][2];
long double runtime_mul = x * y;
if (runtime_mul != build_mul) {
dump_ld("x", x);
dump_ld("y", y);
dump_ld("build_mul", build_mul);
dump_ld("runtime_mul", runtime_mul);
err = 1;
}
}
return err;
}