target/i386: fix fbstp handling of negative zero

The fbstp implementation stores +0 when the rounded result should be
-0 because it compares an integer value with 0 to determine the sign.
Fix this by checking the sign bit of the operand instead.

Signed-off-by: Joseph Myers <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.21.2005132350230.11687@digraph.polyomino.org.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/tests/tcg/i386/test-i386-fbstp.c b/tests/tcg/i386/test-i386-fbstp.c
new file mode 100644
index 0000000000..d368949188
--- /dev/null
+++ b/tests/tcg/i386/test-i386-fbstp.c
@@ -0,0 +1,25 @@
+/* Test fbstp instruction.  */
+
+#include <stdio.h>
+#include <string.h>
+
+int main(void)
+{
+    int ret = 0;
+    unsigned char out[10];
+    memset(out, 0xfe, sizeof out);
+    __asm__ volatile ("fbstp %0" : "=m" (out) : "t" (-0.0L) : "st");
+    out[9] &= 0x80;
+    if (memcmp(out, "\0\0\0\0\0\0\0\0\0\x80", sizeof out) != 0) {
+        printf("FAIL: fbstp -0\n");
+        ret = 1;
+    }
+    memset(out, 0x12, sizeof out);
+    __asm__ volatile ("fbstp %0" : "=m" (out) : "t" (-0.1L) : "st");
+    out[9] &= 0x80;
+    if (memcmp(out, "\0\0\0\0\0\0\0\0\0\x80", sizeof out) != 0) {
+        printf("FAIL: fbstp -0.1\n");
+        ret = 1;
+    }
+    return ret;
+}