Re: [Tinycc-devel] compare problem on riscv target

[Michael Matz]

Hello,

On Thu, 16 Jul 2020, Herman ten Brugge via Tinycc-devel wrote:

> While testing gmp on the riscv target I discovered a compare bug.
> (t-get_d test fails in directory tests/mpq)
>
> The code:
>
> #include <stdio.h>
>
> int tst(void) {
>   long value = 3;
>   return -value;
> }
>
> int main(void) {
>   printf ("%d\n", tst());
>   if (tst() > 0) printf ("error\n");
>   return 0;
> }
>
> prints "error" on the riscv target. The problem is that the value returned is
> converted from long to int in gen_cast and is returned as 0xfffffffd from 
> tst().

On riscv registers should always contain the sign-extended value of 
shorter values, so this here is the bug, and ...

> The compare in the main code fails because it does a long compare
> instead

... this is the okay because of the above invariant.

TCCs other architectures expect either zero extension or don't care or 
natural extension about narrow values in large registers, so riscv was 
somewhat novel in that respect; obviously I didn't catch all corner cases 
yet, so thanks for the report.


Ciao,
Michael.