[Tinycc-devel] Patch for tcc typecast sign extension bug

[David Heine]

I ran into a bug with tcc-0.9.22 related to incorrect sign extension in 
typecasts. I ran into it on the x86 target, but it probably exists for all 
targets.  Here is a testcase that demonstrates the problem:

> cat signext.c
int scast(unsigned v) {
   return (short)v;
}
int ccast(unsigned v) {
   return (signed char)v;
}
int main() {
  if (scast(0xf000) != 0xfffff000) {
    printf("test1: FAIL\n");
  } else {
    printf("test1: PASS\n");
  }
  if (ccast(0xf0f0) != 0xfffffff0) {
    printf("test2: FAIL\n");
  } else {
    printf("test2: PASS\n");
  }
}
> ./tcc -run signext.c
test1: FAIL
test2: FAIL


I tracked this down to the function force_charshort_cast().  When the type on 
the top of the stack is unsigned but it is converting to a signed quantity, it 
generates
   16(or 24) SHL
   16(or 24) SAR
  Because the original value is unsigned, the SHL produces an unsigned value. 
The SAR on the unsigned value is then incorrectly converted to an SHR in 
genop(). My proposed patch forces the result of the SHL to be a signed type when 
casting to a signed value.

I've also included 2 new tests in tcctest.c.

diff -p tcc-0.9.22-orig/tcc.c tcc-0.9.22/tcc.c
*** tcc-0.9.22-orig/tcc.c       Mon Nov  8 12:47:16 2004
--- tcc-0.9.22/tcc.c    Tue Jan 25 14:25:33 2005
*************** void force_charshort_cast(int t)
*** 5613,5618 ****
--- 5613,5623 ----
          bits = 32 - bits;
          vpushi(bits);
          gen_op(TOK_SHL);
+       /* result must be signed or the SAR is converted to an SHL
+          This was not the case when "t" was a signed short
+          and the last value on the stack was an unsigned int
+       */
+       vtop->type.t &= (~VT_UNSIGNED);
          vpushi(bits);
          gen_op(TOK_SAR);
      }
diff -p tcc-0.9.22-orig/tcctest.c tcc-0.9.22/tcctest.c
*** tcc-0.9.22-orig/tcctest.c   Mon Nov  8 12:47:16 2004
--- tcc-0.9.22/tcctest.c        Tue Jan 25 14:17:28 2005
*************** void cast_test()
*** 1058,1063 ****
--- 1058,1065 ----
      int a;
      char c;
      char tab[10];
+     unsigned b,d;
+     short s;
  
      printf("cast_test:\n");
      a = 0xfffff;
*************** void cast_test()
*** 1081,1086 ****
--- 1083,1096 ----
      
      printf("sizeof(c) = %d, sizeof((int)c) = %d\n", sizeof(c), 
sizeof((int)c));
  
+     /* test cast from unsigned to signed short to int */
+     b = 0xf000;
+     d = (short)b;
+     printf("((unsigned)(short)0x%08x) = 0x%08x\n", b, d);
+     b = 0xf0f0;
+     d = (char)b;
+     printf("((unsigned)(char)0x%08x) = 0x%08x\n", b, d);
+ 
      /* test implicit int casting for array accesses */
      c = 0;
      tab[1] = 2;