target/i386: fix fscale handling of infinite exponents

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 <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.21.2005070045010.18350@digraph.polyomino.org.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 29 insertions, 0 deletions

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;
 }