[Qemu-devel] [PATCH] sparc32 : Signed integer division overflow

[Olivier Danet]

I wanted to test an integer divider for SPARC32, and tried the stress 
test program on QEMU, which choked on the division
-0x8000_0000_0000_0000 / -1 (QEMU compiled on x86_64).
Excerpt from the test program :
-------------------------------------------------------------------
#include <stdio.h>

typedef unsigned uint32;
typedef unsigned long long uint64;
typedef signed int32;
typedef signed long long int64;

void sdiv(int64 x, int32 y, int32 *q, uint32 *ov)
{
  uint32 a,b,c,d,e;
  a=x>>32;
  b=x;
  c=y;

  __asm__ __volatile__("wr %2,%%y\n\t"
                       "nop\n\t"
                       "nop\n\t"
                       "nop\n\t"
                       "sdivcc %3,%4,%0\n\t"
                       "or %%r0,0,%1\n\t"
                       "bvc xxa\n\t"
                       "nop\n\t"
                       "add %1,1,%1\n\t"
                       "xxa:bpos xxb\n\t"
                       "nop\n\t"
                       "add %1,2,%1\n\t"
                       "xxb:bne xxc\n\t"
                       "nop\n\t"
                       "add %1,4,%1\n\t"
               "xxc:nop\n\t"
               : "=r"(d),"=r"(e):"r"(a),"r"(b),"r"(c)
               );
  *q=d;
  *ov=e & 1;
}

void main(void)
{
  uint64 x;
  uint32 y;
  int32 q;
  int32 ov;
  x = 1LL << 63;
  y = -1;
  sdiv(x, y, &q, &ov);
}

Here is a patch for handling this corner case on SPARC32.
SPARC64 division already checks this in helper_sdivx(), some other 
architectures
seem to do the same (for example, target-arm/helper.c: HELPER(sdiv))

===================================================================
The integer division 0x8000_0000_0000_0000 / -1 must be handled separately
to avoid overflows on the QEMU host.

Signed-off-by: Olivier Danet <oda...@caramail.com>

-------------------------------------------------------------------
diff --git a/target-sparc/helper.c b/target-sparc/helper.c
index 57c20af..b6b5937 100644
--- a/target-sparc/helper.c
+++ b/target-sparc/helper.c
@@ -116,14 +116,16 @@ static target_ulong 
helper_sdiv_common(CPUSPARCState *env, target_ulong a,
     if (x1 == 0) {
         cpu_restore_state(env, GETPC());
         helper_raise_exception(env, TT_DIV_ZERO);
-    }
-
-    x0 = x0 / x1;
-    if ((int32_t) x0 != x0) {
-        x0 = x0 < 0 ? 0x80000000 : 0x7fffffff;
+    } else if (x1 == -1 && x0 == 0x8000000000000000) {
+        x0 = 0x7fffffff;
         overflow = 1;
+    } else {
+        x0 = x0 / x1;
+        if ((int32_t) x0 != x0) {
+            x0 = x0 < 0 ? 0x80000000 : 0x7fffffff;
+            overflow = 1;
+        }
     }
-
     if (cc) {
         env->cc_dst = x0;
         env->cc_src2 = overflow;
-------------------------------------------------------------------