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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>
This commit is contained in:
Joseph Myers 2020-05-13 23:51:09 +00:00 committed by Paolo Bonzini
parent 34b9cc076f
commit 18c53e1e73
2 changed files with 29 additions and 1 deletions
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5
target/i386/fpu_helper.c
View File

@ -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 {

25
tests/tcg/i386/test-i386-fbstp.c Normal file
View File

@ -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;
}