Re: [Tinycc-devel] Bug in _Bool return values

[Christian Jullien]

Hello Louis,

To help maintainers, can you please give us a complete working example on gcc / VC++ that fails with tcc?

I’ll see how it goes on ARM 32/64.

C.

 

From: Tinycc-devel [mailto:address@hidden On Behalf Of Louis Botha
Sent: lundi 4 février 2019 01:54
To: address@hidden
Subject: [Tinycc-devel] Bug in _Bool return values

 

Hi TCC-fans,

 

Think I found a bug in the implementation of _Bool in return values in version 0.9.27, found on i386, probably applies to x86_64, and possibly others as well:

Since sizeof(_Bool) == 1 as implemented in TCC, I believe this comment in i386-gen.c applies:

“extend the return value to the whole register if necessary

visual studio and gcc do not always set the whole eax register

when assigning the return value of a function”

 

The scenario that broke for me was calling a function that only set the AL register on return, leaving the rest of EAX undefined, yet TCC checked the whole of EAX for the _Bool return value (true came out correct, but false tested as true when the undefined-bits were non-0)

 

Assigning the _Bool return value to a _Bool variable and then checking that variable instead of the return value is one confirmed workaround (the emitted TCC code then uses just AL to generate a full EAX before checking EAX), but I think I have found the proper fix:

 

Add “case VT_BOOL:” as shown here, just after line 365 of i386-gen.c:

 

switch (rt & VT_BTYPE) {

    case VT_BYTE:

    case VT_BOOL:

 

This modification fixed the problem I was having, I *think* the line in x86_64-gen.c that needs to change for the equivalent there is this (@ line 934):

 

else if (bt == VT_BYTE || bt == VT_BOOL)

 

Sorry, I don’t know the other architectures or TCC in general well enough to comment on other places that may also need this fix.

 

Best regards,

Louis