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>

2 files changed, 29 insertions, 1 deletions

diff --git a/target/i386/fpu_helper.c b/target/i386/fpu_helper.c
index 185493db8e..f0a57099ca 100644
--- a/target/i386/fpu_helper.c
+++ b/target/i386/fpu_helper.c
@@ -726,11 +726,14 @@ void helper_fbst_ST0(CPUX86State *env, target_ulong ptr)
     int v;
     target_ulong mem_ref, mem_end;
     int64_t val;
+    CPU_LDoubleU temp;
+
+    temp.d = ST0;
 
     val = floatx80_to_int64(ST0, &env->fp_status);
     mem_ref = ptr;
     mem_end = mem_ref + 9;
-    if (val < 0) {
+    if (SIGND(temp)) {
         cpu_stb_data_ra(env, mem_end, 0x80, GETPC());
         val = -val;
     } else {
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;
+}