Re: An aliasing problem

Hi Bruno,

> * Bruno Haible <[email protected]> [2017-02-26 18:44:58 +0100]:
>
> You wrote in
> <https://sourceforge.net/p/clisp/mailman/message/35687181/>:

Do you have code to get this reference automatically?

>> It dawned on me that this might be an aliasing issue.  The pointer
>> derived from ((unsigned long)STACK[-1-(long)(2+4+2*kept_slots)]-1UL)
>> is visible in this scope twice, once cast to an Instance (pointer to
>> an anonymous struct) and once cast to a Record (record_ *).  I tried
>> building with -fno-strict-aliasing and sure enough, the build
>> succeeded.
>
> Wow! You hit the nail on the head.

Without posting any (or at least not much) assembly, could you please
explain what gcc does with -fstrict-aliasing?

Specifically, I always thought that

float f;
unsigned int i = *(unsigned int *)&f;

is compiled to an inline memcpy or something.
what does gcc do instead when it detects aliasing?

(you can assume that I read http://stackoverflow.com/q/98650/850781 and
http://stackoverflow.com/q/98340/850781 carefully, but no more than that).

> --- a/src/makemake.in   Sun Feb 26 18:27:03 2017 +0100
> +++ b/src/makemake.in   Sun Feb 26 18:30:41 2017 +0100
> @@ -1325,6 +1327,23 @@
>  
>  fi
>  
> +# We access the same memory through pointers of different types, for example
> +# as TheVarobject(obj)->..., TheRecord(obj)->..., TheInstance(obj)->... .
> +# This violates the strict type-based aliasing rules of C. In other words, we
> +# still use C as a portable assembler, but now the compilers want to outsmart
> +# us. There are two ways to tell them not to do this: to use union types, or
> +# specific compiler options. I prefer to do it through compiler options.
> +if [ $XCC_GCC = true ] ; then
> +  XCFLAGS=$XCFLAGS' -fno-strict-aliasing'
> +else
> +  if [ "$HSYSOS" = aix ] ; then # for xlc
> +    XCFLAGS=$XCFLAGS' -qalias=noansi'
> +  fi
> +  if [ -n "$XCC_SUNPRO" ] ; then # for SUNWspro cc
> +    XCFLAGS=$XCFLAGS' -xalias_level=weak'
> +  fi
> +fi
> +
>  if [ "${with_dynamic_modules}" != no ]; then # not on msvc
>    # Support for dynamic loading.
>    eval "`./libtool --tag=CC --config | grep '^pic_flag='`"

shouldn't it be something like

--8<---------------cut here---------------start------------->8---
diff -r 208ade5c97ca src/makemake.in
--- a/src/makemake.in	Sun Feb 26 20:07:47 2017 -0500
+++ b/src/makemake.in	Mon Feb 27 13:05:45 2017 -0500
@@ -1335,13 +1335,12 @@ fi
 # specific compiler options. I prefer to do it through compiler options.
 if [ $XCC_GCC = true ] ; then
   XCFLAGS=$XCFLAGS' -fno-strict-aliasing'
-else
-  if [ "$HSYSOS" = aix ] ; then # for xlc
+elif [ "$HSYSOS" = aix ] ; then # for xlc
     XCFLAGS=$XCFLAGS' -qalias=noansi'
-  fi
-  if [ -n "$XCC_SUNPRO" ] ; then # for SUNWspro cc
+elif [ -n "$XCC_SUNPRO" ] ; then # for SUNWspro cc
     XCFLAGS=$XCFLAGS' -xalias_level=weak'
-  fi
+elif [ "${with_debug}" != no ]; then
+  echo "$0: WARNING: how to disable strict aliasing with ${XCC} on ${HSYSOS}?" >&2
 fi
 
 if [ "${with_dynamic_modules}" != no ]; then # not on msvc
--8<---------------cut here---------------end--------------->8---




-- 
Sam Steingold (http://sds.podval.org/) on darwin Ns 10.3.1504
http://steingoldpsychology.com http://www.childpsy.net http://memri.org
http://think-israel.org http://camera.org http://www.dhimmitude.org
He who laughs last did not get the joke.

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