[Vm-dev] abs() generation problem
Esteban Lorenzano
estebanlm at gmail.com
Fri Mar 25 16:24:52 UTC 2016
which would not be an issue if we were using git… :)
(sorry, I couldn’t resist it)
Esteban
> On 25 Mar 2016, at 16:58, David T. Lewis <lewis at mail.msen.com> wrote:
>
>
> +1 to adding Nicolas
>
> (I might not be able to do it for a couple of days, please be patient)
>
> Dave
>
> On Thu, Mar 24, 2016 at 10:28:04PM -0700, John McIntosh wrote:
>>
>> Fine by me
>>
>> On Thu, Mar 24, 2016 at 10:10 PM, Eliot Miranda <eliot.miranda at gmail.com>
>> wrote:
>>
>>>
>>> Dave,
>>>
>>> can we give Nicolas write permission to the svn repository please? At
>>> least the cog branch but I think he's to be trusted elsewhere too. Do
>>> others agree?
>>>
>>> _,,,^..^,,,_ (phone)
>>>
>>>> On Mar 24, 2016, at 4:28 PM, Nicolas Cellier <
>>> nicolas.cellier.aka.nice at gmail.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> abs(sqInt) is a problem on 64bits system.
>>>> Because sqInt is a long, this should be labs(sqInt).
>>>>
>>>> Well, as long as we only use 32 bits out of the 64, AND restrict ourself
>>> to little endian, this wrong code apparently works.
>>>> But it's so fragile:
>>>>
>>>> #cat > test_abs.c <<END
>>>> #include <stdio.h>
>>>> int main() {
>>>> long long x=(1LL << 33) - 1;
>>>> long long y = abs(x);
>>>> printf("x=%lld abs(x)=%lld\n",x,y);
>>>> }
>>>> END
>>>>
>>>> #clang test_abs.c
>>>> test_abs.c:4:17: warning: implicitly declaring library function 'abs'
>>> with type 'int (int)'
>>>> [-Wimplicit-function-declaration]
>>>> long long y = abs(x);
>>>> ^
>>>> test_abs.c:4:17: note: include the header <stdlib.h> or explicitly
>>> provide a declaration for 'abs'
>>>> test_abs.c:4:17: warning: absolute value function 'abs' given an
>>> argument of type 'long long' but has parameter of type
>>>> 'int' which may cause truncation of value [-Wabsolute-value]
>>>> long long y = abs(x);
>>>> ^
>>>> test_abs.c:4:17: note: use function 'llabs' instead
>>>> long long y = abs(x);
>>>> ^~~
>>>> llabs
>>>> test_abs.c:4:17: note: include the header <stdlib.h> or explicitly
>>> provide a declaration for 'llabs'
>>>>
>>>> #./a.out
>>>> x=8589934591 abs(x)=1
>>>>
>>>>
>>>> So we can think of adding a #generateAbs:on:indent:
>>>> and handle the #typeFor:in:
>>>>
>>>> BUT: sqInt is either defined as int or long depending on the target:
>>>>
>>>> #if defined(SQ_IMAGE32)
>>>> typedef int sqInt;
>>>> typedef unsigned int usqInt;
>>>> #elif defined(SQ_HOST64)
>>>> typedef long sqInt;
>>>> typedef unsigned long usqInt;
>>>> #elif (SIZEOF_LONG_LONG != 8)
>>>> # error long long integers are not 64-bits wide?
>>>> #else
>>>> typedef long long sqInt;
>>>> typedef unsigned long long usqInt;
>>>> #endif
>>>>
>>>> Here, I propose 3 solutions:
>>>> 1) since sqInt=int=long in 32bits spur, and sqInt=long in 64bits spur we
>>> can cheat:
>>>> sqLong 'long long' int64_t __int64 -> #llabs
>>>> sqInt long -> #lasb
>>>> int int32_t __int32 -> #abs
>>>> it's not straight and may still generate compiler warnings, which
>>> should be avoided
>>>> 2) testing the wordSize, and branching in the generator:
>>>> is64bitsVM := vmClass notNil and: [vmClass objectMemoryClass
>>> wordSize = 8].
>>>> Ah no, we currently generate single source for src/plugins in 32 and
>>> 64bits flavour,
>>>> that ain't gonna work, or we should differentiate the plugin
>>> generation
>>>> 3) generate a macro SQABS for sqInt and define it in sqMemoryAccess.h,
>>> same of sqLong
>>>> sqInt -> SQABS
>>>> sqLong -> SQLABS
>>>>
>>>> The correct solution is 3), but I have no commit right to svn, so
>>> someone should do it.
>>>> I'm attaching my own sqMemoryAccess.h copy, but please note that it has
>>> other improvments like
>>>> - avoiding un-strict pointer aliasing via memcpy for fetching/storing
>>> float (except swapper in BIGENDIAN branch)
>>>> - providing some native unsigned memory access
>>>>
>>>> ----------------------------------------------
>>>>
>>>> Now what about unsigned types?
>>>>
>>>> #cat > test_abs.c <<END
>>>> #include <stdio.h>
>>>> int main() {
>>>> unsigned int x=-1;
>>>> printf("abs(x)=%d\n",abs(x));
>>>> return 0;
>>>> }
>>>> END
>>>>
>>>> #test_abs.c:4:24: warning: implicitly declaring library function 'abs'
>>> with type 'int (int)'
>>>> [-Wimplicit-function-declaration]
>>>> printf("abs(x)=%d\n",abs(x));
>>>> ^
>>>> test_abs.c:4:24: note: include the header <stdlib.h> or explicitly
>>> provide a declaration for 'abs'
>>>> test_abs.c:4:24: warning: taking the absolute value of unsigned type
>>> 'unsigned int' has no effect [-Wabsolute-value]
>>>> printf("abs(x)=%d\n",abs(x));
>>>> ^
>>>> test_abs.c:4:24: note: remove the call to 'abs' since unsigned values
>>> cannot be negative
>>>> printf("abs(x)=%d\n",abs(x));
>>>> ^~~
>>>>
>>>> #./a.out
>>>> abs(x)=1
>>>>
>>>> clang first tells that there is no point of taking absolute value of an
>>> unsigned int, an unsigned is allways positive.
>>>>
>>>> BUT, then it re-interprets the unsigned as signed, and the program
>>> return +1.
>>>> It would return -1 if abs were really a no-op.
>>>>
>>>> So there are several alternatives for the generator here:
>>>> - 1) suppress the generation of abs for an unsigned
>>>> BEWARE, this will change behaviour vs current state of VMMaker
>>>> - 2) force a signed cast to remove the warning
>>>> we just enforce current state of VMMaker but tell the compiler to
>>> shut up
>>>> - 3) do nothing and leave the problem for later...
>>>> the compiler will spit more errors
>>>>
>>>> I'm attaching solution 3), have programmed 2) in my own branch.
>>>> 1) is dangerous because of 2 things :
>>>> - unwanted promotion to unsigned type in C - simply put a sizeof(...) in
>>> the expression and it turns unsigned...
>>>> - limited confidence about how slang inlining is handling the implicit
>>> type conversions...
>>>>
>>>> Currently there is at least 1 abs(unsigned) which is generated in
>>> shrinkObjectMemory :
>>>> ../../spurstack64src/vm/gcc3x-interp.c:50178:12: warning: taking the
>>> absolute value of unsigned type 'unsigned long' has no effect
>>> [-Wabsolute-value]
>>>> && ((SQABS((((seg->segSize)) -
>>> shrinkage))) < delta1)) {
>>>>
>>>> This is because shrinkage is declared usqInt...
>>>>
>>>> There would be yet another solution
>>>> - 4) raise an exception and let the dear VMMaker user handle the
>>> ambiguity
>>>>
>>>> That's my favourite, but It's only a comment in attachment, I don't want
>>> to break yet another Jenkins job ;)
>>>> We should force the correct cast where there is ambiguity.
>>>> If we do not, the exception could be proceedable and would proceed to
>>> solution 3) -
>>>> (we do not shut up the compiler and give another chance to someone to
>>> understand and correct slang source...)
>>>>
>>>> Eliot, David, Tim, others, what do you think ?
>>>>
>>>> <sqMemoryAccess.h>
>>>> <patch_abs_generation.st>
>>>
>>
>>
>>
>> --
>> ===========================================================================
>> John M. McIntosh. Corporate Smalltalk Consulting Ltd
>> https://www.linkedin.com/in/smalltalk
>> ===========================================================================
>
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