Re: [PATCH] Fix PR60319, mixing -f[no-]strict-overflow units with LTO

[Jan Hubicka <hubicka at ucw dot cz>]

> 
> This addresses miscompilations caused by improperly mixing
> -f[no-]strict-overflow units (and thus optimize <= 1 and optimize >= 2
> units!).  It does so by merging -f[no-]strict-overflow and the
> related -f[no-]wrapv and -f[no-]trapv conservatively, that is,
> if any unit uses (implicitely) -fno-strict-overflow, -fwrapv
> or -fno-trapv then force that upon the compilation in and after
> WPA stage.
> 
> I know honza wants to eventually handle this via optimize attributes
> but currently at least -fstrict-overflow is not suitable for that
> (it's not marked 'Optimization').  Nor do we even check for
> -f[no-]wrapv mismatch in can_inline_edge_p.

Yep, we need to fix all that for 4.10
> 
> LTO bootstrap & test ongoing on x86_64-unknown-linux-gnu, ok for trunk?
> 
> Any other "obvious" candidates for conservative merging?
> 
> Note that this makes it quite harmful (well - maybe?) if your
> LTO compile contains a single non-optimized TU ... but you
> at least know if you inspect the lto_opts section of one of the
> ltrans files.

Indeed, i think we should at least mention that in news.html and perhsps
have short section on LTO and flags in the doc/invoke.texi?
Users generally get wrong the fact that they need -flto and optimization
flags at command line and we do crazy things behind their back.
Like last week I noticed that we build a static binary of Firefox's shell
with -fpic just because they manage to merge in an object file copiled -fPIC.
(while LLVM give non-PIC binary)
-fno-pic helps there, what will bebehaviour with -fstrict-overflow?
> 
> Eventually this asks for a diagnostic by lto-wrapper (but that
> gets delayed until very late ...).

I am definitely trying to push myself to make command line options an priority
for 4.10 stage1. Lets try to wrok on it, I think it is one of main sowstoppers for
LTO at the moment. (along with debug info and fun with ASM statements definitng
symbols I suppose)

Honza
> 
> Thanks,
> Richard.
> 
> 2014-02-24  Richard Biener  <rguenther@suse.de>
> 
> 	PR lto/60319
> 	* lto-opts.c (lto_write_options): Output non-explicit conservative
> 	-fwrapv, -fno-trapv and -fno-strict-overflow.
> 	* lto-wrapper.c (merge_and_complain): Handle merging those options.
> 	(run_gcc): And pass them through.
> 
> Index: gcc/lto-opts.c
> ===================================================================
> *** gcc/lto-opts.c	(revision 208077)
> --- gcc/lto-opts.c	(working copy)
> *************** lto_write_options (void)
> *** 117,122 ****
> --- 117,134 ----
>         default:
>   	gcc_unreachable ();
>         }
> +   /* We need to merge -f[no-]strict-overflow, -f[no-]wrapv and -f[no-]trapv
> +      conservatively, so stream out their defaults.  */
> +   if (!global_options_set.x_flag_wrapv
> +       && global_options.x_flag_wrapv)
> +     append_to_collect_gcc_options (&temporary_obstack, &first_p, "-fwrapv");
> +   if (!global_options_set.x_flag_trapv
> +       && !global_options.x_flag_trapv)
> +     append_to_collect_gcc_options (&temporary_obstack, &first_p, "-fno-trapv");
> +   if (!global_options_set.x_flag_strict_overflow
> +       && !global_options.x_flag_strict_overflow)
> +     append_to_collect_gcc_options (&temporary_obstack, &first_p,
> + 			       "-fno-strict-overflow");
>   
>     /* Output explicitly passed options.  */
>     for (i = 1; i < save_decoded_options_count; ++i)
> Index: gcc/lto-wrapper.c
> ===================================================================
> *** gcc/lto-wrapper.c	(revision 208077)
> --- gcc/lto-wrapper.c	(working copy)
> *************** merge_and_complain (struct cl_decoded_op
> *** 422,427 ****
> --- 422,429 ----
>   	    append_option (decoded_options, decoded_options_count, foption);
>   	  break;
>   
> + 	case OPT_ftrapv:
> + 	case OPT_fstrict_overflow:
>   	case OPT_ffp_contract_:
>   	  /* For selected options we can merge conservatively.  */
>   	  for (j = 0; j < *decoded_options_count; ++j)
> *************** merge_and_complain (struct cl_decoded_op
> *** 429,439 ****
>   	      break;
>   	  if (j == *decoded_options_count)
>   	    append_option (decoded_options, decoded_options_count, foption);
> ! 	  /* FP_CONTRACT_OFF < FP_CONTRACT_ON < FP_CONTRACT_FAST.  */
>   	  else if (foption->value < (*decoded_options)[j].value)
>   	    (*decoded_options)[j] = *foption;
>   	  break;
>   
>   	case OPT_freg_struct_return:
>   	case OPT_fpcc_struct_return:
>   	  for (j = 0; j < *decoded_options_count; ++j)
> --- 431,455 ----
>   	      break;
>   	  if (j == *decoded_options_count)
>   	    append_option (decoded_options, decoded_options_count, foption);
> ! 	  /* FP_CONTRACT_OFF < FP_CONTRACT_ON < FP_CONTRACT_FAST,
> ! 	     -fno-trapv < -ftrapv,
> ! 	     -fno-strict-overflow < -fstrict-overflow  */
>   	  else if (foption->value < (*decoded_options)[j].value)
>   	    (*decoded_options)[j] = *foption;
>   	  break;
>   
> + 	case OPT_fwrapv:
> + 	  /* For selected options we can merge conservatively.  */
> + 	  for (j = 0; j < *decoded_options_count; ++j)
> + 	    if ((*decoded_options)[j].opt_index == foption->opt_index)
> + 	      break;
> + 	  if (j == *decoded_options_count)
> + 	    append_option (decoded_options, decoded_options_count, foption);
> + 	  /* -fwrapv > -fno-wrapv.  */
> + 	  else if (foption->value > (*decoded_options)[j].value)
> + 	    (*decoded_options)[j] = *foption;
> + 	  break;
> + 
>   	case OPT_freg_struct_return:
>   	case OPT_fpcc_struct_return:
>   	  for (j = 0; j < *decoded_options_count; ++j)
> *************** run_gcc (unsigned argc, char *argv[])
> *** 591,596 ****
> --- 607,615 ----
>   	case OPT_freg_struct_return:
>   	case OPT_fpcc_struct_return:
>   	case OPT_ffp_contract_:
> + 	case OPT_fwrapv:
> + 	case OPT_ftrapv:
> + 	case OPT_fstrict_overflow:
>   	  break;
>   
>   	default: