Re: make 3.82 considered harmful

[Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>]

On 13-06-2013 15:26, Roland McGrath wrote:
> The recent build issues are more fallout of the extremely ill-advised
> change to the 20-year-old semantics of GNU make's pattern rules that was
> made in version 3.82 (despite my strenuous objections).  It's described
> like this in make's NEWS:
>
> * WARNING: Backward-incompatibility!
>   The pattern-specific variables and pattern rules are now applied in the
>   shortest stem first order instead of the definition order (variables
>   and rules with the same stem length are still applied in the definition
>   order). This produces the usually-desired behavior where more specific
>   patterns are preferred. To detect this feature search for 'shortest-stem'
>   in the .FEATURES special variable.
>
> (In fact, 3.82 has seven NEWS items marked with the same scary line,
> "WARNING: Backward-incompatibility!"  This alone is a clear indicator that
> this was a thoroughly ill-conceived release.)
>
> This change breaks the fundamental design of our build system, in
> particular sysd-rules.  It's crucial that the rules there be applied in an
> exact order that we choose.  With the 3.82 change in these long-established
> semantics, it's now rather hard to figure out which rule will be chosen.
>
> Most libc hackers are still using 3.81, but Adhemerval's recent change to
> sysdeps/generic/symbol-hacks.h was tested (and works) only with 3.82.  This
> has uncovered the fact that somehow we changed things at some point so that
> at least the rtld-* rules in sysd-rules are only working right under 3.82.
>
> We now have various source files called rtld-*.[cS] in places in the
> sysdeps/ tree.  rtld-memset.os is built from sysdeps/x86_64/rtld-memset.c
> but this is now happening (under 3.81) with the wrong rule, so it does not
> include $(rtld-CPPFLAGS).  This has been happening for a long time, but we
> never noticed before because it didn't happen to matter in practice.
> Adhemerval's recent change makes it matter, but since he was using 3.82 he
> didn't notice.
>
> I used this temporary hack to figure out which rule was being chosen:
>
> --- a/Makerules
> +++ b/Makerules
> @@ -238,6 +238,7 @@ $(common-objpfx)sysd-rules: $(common-objpfx)config.make $(..)Makerules \
>  			    $(sysd-rules-force)
>  	-@rm -f $@T
>  	(echo 'sysd-rules-sysdirs := $(config-sysdirs)';		      \
> +	 n=0; \
>  	 for dir in $(config-sysdirs); do				      \
>  	   case "$$dir" in						      \
>  	   /*) ;;							      \
> @@ -253,6 +254,8 @@ $(common-objpfx)sysd-rules: $(common-objpfx)config.make $(..)Makerules \
>  	       v=$${t%%%}; [ x"$$v" = x ] || v="\$$($${v}CPPFLAGS)";	      \
>  	       for s in $$asm .c; do					      \
>  		 echo "\$$(objpfx)$$t$$o: $$dir/$$d$$s \$$(before-compile)";  \
> +		 n=`expr $$n + 1`; \
> +		 echo "	echo rule $$n"; \
>  		 echo "	\$$(compile-command$$s) $$v";			      \
>  	       done;							      \
>  	     done;							      \
>
> That told me rtld-memset.os was built by rule 1462:
>
> $(objpfx)%.os: $(..)sysdeps/x86_64/%.c $(before-compile)
> 	echo rule 1462
> 	$(compile-command.c) 
>
> The one we want is a slightly later one:
>
> $(objpfx)rtld-%.os: $(..)sysdeps/x86_64/rtld-%.c $(before-compile)
> 	echo rule 1464
> 	$(compile-command.c) $(rtld-CPPFLAGS)
>
> So the sysd-rules rule in Makerules needs to sort sysd-rules-patterns by
> length or something like that.  I'll look into it further after lunch.
>
>
> Thanks,
> Roland
>
Thanks for digging into this, I was progressing slower than I expected.