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[PATCH 4/5] target/i386: fix fscale handling of infinite exponents
From: |
Joseph Myers |
Subject: |
[PATCH 4/5] target/i386: fix fscale handling of infinite exponents |
Date: |
Thu, 7 May 2020 00:45:38 +0000 |
User-agent: |
Alpine 2.21 (DEB 202 2017-01-01) |
The fscale implementation passes infinite exponents through to generic
code that rounds the exponent to a 32-bit integer before using
floatx80_scalbn. In round-to-nearest mode, and ignoring exceptions,
this works in many cases. But it fails to handle the special cases of
scaling 0 by a +Inf exponent or an infinity by a -Inf exponent, which
should produce a NaN, and because it produces an inexact result for
finite nonzero numbers being scaled, the result is sometimes incorrect
in other rounding modes. Add appropriate handling of infinite
exponents to produce a NaN or an appropriately signed exact zero or
infinity as a result.
Signed-off-by: Joseph Myers <address@hidden>
---
target/i386/fpu_helper.c | 22 ++++++++++++++++++++++
tests/tcg/i386/test-i386-fscale.c | 29 +++++++++++++++++++++++++++++
2 files changed, 51 insertions(+)
diff --git a/target/i386/fpu_helper.c b/target/i386/fpu_helper.c
index 7709af8fdd..d4c15728e1 100644
--- a/target/i386/fpu_helper.c
+++ b/target/i386/fpu_helper.c
@@ -977,6 +977,28 @@ void helper_fscale(CPUX86State *env)
float_raise(float_flag_invalid, &env->fp_status);
ST0 = floatx80_silence_nan(ST0, &env->fp_status);
}
+ } else if (floatx80_is_infinity(ST1) &&
+ !floatx80_invalid_encoding(ST0) &&
+ !floatx80_is_any_nan(ST0)) {
+ if (floatx80_is_neg(ST1)) {
+ if (floatx80_is_infinity(ST0)) {
+ float_raise(float_flag_invalid, &env->fp_status);
+ ST0 = floatx80_default_nan(&env->fp_status);
+ } else {
+ ST0 = (floatx80_is_neg(ST0) ?
+ floatx80_chs(floatx80_zero) :
+ floatx80_zero);
+ }
+ } else {
+ if (floatx80_is_zero(ST0)) {
+ float_raise(float_flag_invalid, &env->fp_status);
+ ST0 = floatx80_default_nan(&env->fp_status);
+ } else {
+ ST0 = (floatx80_is_neg(ST0) ?
+ floatx80_chs(floatx80_infinity) :
+ floatx80_infinity);
+ }
+ }
} else {
int n = floatx80_to_int32_round_to_zero(ST1, &env->fp_status);
ST0 = floatx80_scalbn(ST0, n, &env->fp_status);
diff --git a/tests/tcg/i386/test-i386-fscale.c
b/tests/tcg/i386/test-i386-fscale.c
index b65a055d0a..b953e7c563 100644
--- a/tests/tcg/i386/test-i386-fscale.c
+++ b/tests/tcg/i386/test-i386-fscale.c
@@ -31,6 +31,7 @@ int issignaling_ld(long double x)
int main(void)
{
+ short cw;
int ret = 0;
__asm__ volatile ("fscale" : "=t" (ld_res) :
"0" (2.5L), "u" (__builtin_nansl("")));
@@ -62,5 +63,33 @@ int main(void)
printf("FAIL: fscale invalid 4\n");
ret = 1;
}
+ __asm__ volatile ("fscale" : "=t" (ld_res) :
+ "0" (0.0L), "u" (__builtin_infl()));
+ if (!isnan_ld(ld_res) || issignaling_ld(ld_res)) {
+ printf("FAIL: fscale 0 up inf\n");
+ ret = 1;
+ }
+ __asm__ volatile ("fscale" : "=t" (ld_res) :
+ "0" (__builtin_infl()), "u" (-__builtin_infl()));
+ if (!isnan_ld(ld_res) || issignaling_ld(ld_res)) {
+ printf("FAIL: fscale inf down inf\n");
+ ret = 1;
+ }
+ /* Set round-downward. */
+ __asm__ volatile ("fnstcw %0" : "=m" (cw));
+ cw = (cw & ~0xc00) | 0x400;
+ __asm__ volatile ("fldcw %0" : : "m" (cw));
+ __asm__ volatile ("fscale" : "=t" (ld_res) :
+ "0" (1.0L), "u" (__builtin_infl()));
+ if (ld_res != __builtin_infl()) {
+ printf("FAIL: fscale finite up inf\n");
+ ret = 1;
+ }
+ __asm__ volatile ("fscale" : "=t" (ld_res) :
+ "0" (-1.0L), "u" (-__builtin_infl()));
+ if (ld_res != -0.0L || __builtin_copysignl(1.0L, ld_res) != -1.0L) {
+ printf("FAIL: fscale finite down inf\n");
+ ret = 1;
+ }
return ret;
}
--
2.17.1
--
Joseph S. Myers
address@hidden
- [PATCH 0/5] target/i386: fxtract, fscale fixes, Joseph Myers, 2020/05/06
- [PATCH 1/5] target/i386: implement special cases for fxtract, Joseph Myers, 2020/05/06
- [PATCH 2/5] target/i386: fix fscale handling of signaling NaN, Joseph Myers, 2020/05/06
- [PATCH 3/5] target/i386: fix fscale handling of invalid exponent encodings, Joseph Myers, 2020/05/06
- [PATCH 4/5] target/i386: fix fscale handling of infinite exponents,
Joseph Myers <=
- [PATCH 5/5] target/i386: fix fscale handling of rounding precision, Joseph Myers, 2020/05/06
- Re: [PATCH 0/5] target/i386: fxtract, fscale fixes, no-reply, 2020/05/06
- Ping Re: [PATCH 0/5] target/i386: fxtract, fscale fixes, Joseph Myers, 2020/05/14
- Re: [PATCH 0/5] target/i386: fxtract, fscale fixes, Philippe Mathieu-Daudé, 2020/05/15
- Re: [PATCH 0/5] target/i386: fxtract, fscale fixes, Paolo Bonzini, 2020/05/21