[Tinycc-devel] Help! - bug in tinycc/tcc - (int8_t) not sign-extended to an unsigned, unless first placed in temporary

[David A. Wheeler]

Hi - I think I've found a bug in tinycc, and help (esp. a patch!!)
would be really appreciated.  Below is a really simple (repeatable!)
test program, along with expected & actual results.

The basic problem is that if "num" is an unsigned (e.g., uint32_t),
I believe an expression like this:
 num == (int8_t) num
should be true if num is 1 or 0xffffffdc,
and NOT true if num is 0xfe or such. gcc does this.
tinycc (at least 0.9.22) also does this if you insert
an extra (unnecessary) temporary variable as a workaround.
But tcc does NOT compute the same result without the temp variable.
Semantics about conversions are complex anyway, but I think
it's clear that inserting a temp variable should NOT change an
answer, and I believe tcc is doing the wrong thing here
without the temp variable.

There appears to be some bug in the tcc conversion code.

Can anyone suggest a patch to tcc for this?

--- David A. Wheeler







/// TEST CODE
/* Demo of tcc bug */

#include <stdio.h>
#include <inttypes.h>

main() {
// This shows what the current tcc code does.
// 1,0,1 is the expected answer
// and is what gcc produces; but tcc produces 1,1,0.
// tcc 0.9.22 doesn't properly do sign-extension from
// a value cast to int8_t
       uint32_t num;
       int8_t tmp;

       printf("Testing tcc and gcc\n");
       num = 1;
       printf("0x%x: result=%d (correct=1)\n", (int) num, num == 
(int8_t) num);
       num = 0xfe;
       printf("0x%x: result=%d (correct=0)\n", (int) num, num == 
(int8_t) num);
       num = 0xffffffdc;
       printf("0x%x: result=%d (correct=1)\n", (int) num, num == 
(int8_t) num);

// By inserting an extra temporary variable,
// the tcc bug is worked around
       printf("\nTesting workaround for tcc (excess temporary var)\n");
       num = 1;
       tmp = (int8_t) num;
       printf("0x%x: result=%d (correct=1)\n", (int) num, num == tmp);
       num = 0xfe;
       tmp = (int8_t) num;
       printf("0x%x: result=%d (correct=0)\n", (int) num, num == tmp);
       num = 0xffffffdc;
       tmp = (int8_t) num;
       printf("0x%x: result=%d (correct=1)\n", (int) num, num == tmp);
}
/// END OF TEST CODE


*** Here's what gcc does ***

Testing tcc and gcc
0x1: result=1 (correct=1)
0xfe: result=0 (correct=0)
0xffffffdc: result=1 (correct=1)

Testing workaround for tcc (excess temporary var)
0x1: result=1 (correct=1)
0xfe: result=0 (correct=0)
0xffffffdc: result=1 (correct=1)


*** Here's what tcc 0.9.22 does ***
Testing tcc and gcc
0x1: result=1 (correct=1)
0xfe: result=1 (correct=0)
0xffffffdc: result=0 (correct=1)

Testing workaround for tcc (excess temporary var)
0x1: result=1 (correct=1)
0xfe: result=0 (correct=0)
0xffffffdc: result=1 (correct=1)