[Vm-dev] abs() generation problem
John McIntosh
johnmci at smalltalkconsulting.com
Fri Mar 25 05:28:04 UTC 2016
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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