Re: [PATCH][TRIVIAL] Remove bogus "value 0x37ffffff truncated to0x37ffffff" warning.

[Bart Samwel]

Davide Libenzi wrote:
> > > #define MAXMEM                       (~__PAGE_OFFSET + 1 - __VMALLOC_RESERVE)
> >
> > I tried that first, before I came up with the solution in the patch, 
> > because I didn't like the dependency of 0xFFFFFFFF being 32-bit. It was 
> > a nice idea, but it didn't work. Apparently, gas interprets ~ as a one's 
> > complement negation operator, not a bitwise or. Therefore, 
> > ~__PAGE_OFFSET is just as negative as -__PAGE_OFFSET as far as gas is 
> > concerned. It gives me the same warning.
>
>
> That would mean a bug in as. __PAGE_OFFSET is unsigned and ~ is documented 
> (not a surprise) as "bitwise not". The bitwise not of __PAGE_OFFSET 
> (unsigned) is still unsigned. BTW 2.14 does not give warnings with both 
> the original statement and the ~ one. This:
>
>                                                                                                                         
>         PG=0xC0000000                                                                                                   
>         VM=(128 << 20)                                                                                                  
>                                                                                                                         
>         mov (~PG + 1 - VM), %eax                                                                                        
>         mov (-PG - VM), %eax                                                                                            
>                                                                                                                         
> generate this:
>
> zzzzzzzz:     file format elf32-i386
>
> Disassembly of section .text:
>
> 00000000 <.text>:
>    0:   a1 00 00 00 38          mov    0x38000000,%eax
>    5:   a1 00 00 00 38          mov    0x38000000,%eax
>
>
> w/out any warnings. And the result is obviously 0x38000000 and 
> not 0x37ffffff.

I get the same behaviour. The 0x37ffffff is from the place where MAXMEM 
is used (the ramdisk_max variable in setup.S); it subtracts one from the 
value. It turns out that the error only occurs when the value is used in 
a data definition. Experimentally found first value for which it gives 
the error is:

ramdisk_max: .long ~(0x80000000)

Interestingly, it doesn't occur for 0x7fffffff. I've taken a look at gas 
to see where it goes wrong, but my newly built version doesn't exhibit 
this behaviour -- it compiles the above statement without warnings. It 
might have to do with the differences between the build environment that 
the Debian binutils package is built in and my own machine -- I'll do 
some more investigating.

-- Bart
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