Always pass -fno-strict-aliasing during compilation

[JohnoKing]

The ksh93 codebase is incompatible with strict aliasing, yet currently the default flags enable -fstrict-aliasing via the -Os optimization flag. Additionally, manually passing a higher level optimization flag via CCFLAGS (e.g. -O3) will result in strict aliasing becoming inadvertently enabled unless -fno-strict-aliasing is explicitly passed to disable it (cf. the Fedora and OpenSUSE build scripts for ksh*).

This commit implements a probe that will detect the existence of -fno-strict-aliasing and use it via ${mam_cc_FLAGS}, bypassing CCFLAGS.

References:
https://src.fedoraproject.org/rpms/ksh/blob/rawhide/f/ksh.spec#_49
https://build.opensuse.org/package/view_file/shells/ksh/ksh.spec?expand=1

[McDutchie]

I have been compiling and running ksh with -Os with no problems for years. Is it actually true that this code base is incompatible with strict aliasing? Which bits are incompatible, and can we fix them?

I need to learn more about what this means, but I need to find the time to do that first.

[JohnoKing]

Most of the strict aliasing violations in the codebase are undetected because -Wstrict-aliasing is set to the lowest setting by default, for the purpose of avoiding false positives. However, raising the level to -Wstrict-aliasing=2 is enough to catch many examples of actual strict aliasing violations, such as this one:

/home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/bltins/print.c: In function 'B_echo':
/home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/bltins/print.c:119:51: warning: dereferencing type-punned pointer might break strict-aliasing rules [-Wstrict-aliasing]
  119 |                 return b_print(0,argv,(Shbltin_t*)&prdata);
      |                                                   ^~~~~~~
/home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/bltins/print.c:139:43: warning: dereferencing type-punned pointer might break strict-aliasing rules [-Wstrict-aliasing]
  139 |         return b_print(0,argv,(Shbltin_t*)&prdata);
      |                                           ^~~~~~~
/home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/bltins/print.c: In function 'b_printf':
/home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/bltins/print.c:150:44: warning: dereferencing type-punned pointer might break strict-aliasing rules [-Wstrict-aliasing]
  150 |         return b_print(-1,argv,(Shbltin_t*)&prdata);
      |                                            ^~~~~~~

ksh/src/cmd/ksh93/bltins/print.c

Lines 105 to 107 in 3bbe308

	struct print prdata;
	prdata.options = sh_optecho+5;
	prdata.raw = prdata.echon = 0;

ksh/src/cmd/ksh93/bltins/print.c

Line 139 in 3bbe308

return b_print(0,argv,(Shbltin_t*)&prdata);

ksh/src/cmd/ksh93/bltins/print.c

Line 189 in 3bbe308

struct print *pp = (struct print*)context;

Here, prdata, which is struct print, is cast to a Shbltin_t so that it can be passed as an argument to b_print. Many more of the warnings produced by -Wstrict-aliasing=2 are likely indicative of strict aliasing violations, and that's not even the highest warning level!

Build log with the strict aliasing warning level set to 1: log.txt