Re: [Tinycc-devel] lcall invalid hex code

[Michael Matz]

Hi,

On Mon, 2 May 2016, Sergey Korshunoff wrote:

> fix the "bug #39341: TCC emits wrong opcode for XCHG"
> ....
>    #include <stdio.h>
>    int main() {
>        int i = 0x16789;
>        printf("Number:   %x\n", i);
>        __asm__ __volatile__ ("xchg %%ah, %%al;" : "=a" (i) : "0" (i));
>        printf("Reversed: %x\n", i);
>    }
>    // The asm instruction is 0x94 which is xchg %eax,%esp (according to objdump
>    // and ollydbg). This leads to an immediate segfault!
>    // The correct opcode would be 0x86 0xE0..
>
>    A bug description: a byte code ops must be declared _before_
>    a word codes in i386-asm.h and x86_64-asm.h

Not really.  It does work-around the above problem, but at the expense of 
not using the one-byte opcode 0x90+reg anymore, even if possible. 
Generally the opcode in i386-asm.h are sorted by some sort of preference 
(e.g. first the one-byte opcodes for nops, then the more capable two-byte 
opcodes for general xchg).  Something isn't right in the i386-asm.c opcode 
parser/selector.  It should reject the first ("xchgw") alternative if 
presented with byte-registers.  I'll take a look.


Ciao,
Michael.