[PULL,054/115] target/i386: fix fscale handling of infinite exponents

Commit Message

Paolo Bonzini June 11, 2020, 7:43 p.m. UTC

From: Joseph Myers <[e-mail]>

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 <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>
---
 target/i386/fpu_helper.c          | 22 ++++++++++++++++++++++
 tests/tcg/i386/test-i386-fscale.c | 29 +++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

Patch

diff --git a/target/i386/fpu_helper.c b/target/i386/fpu_helper.c
index 10ff90370e..72d1e77eb0 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;
 }