Re: [PATCH] clarify documentation of -Q --help=optimizers

[Martin Sebor <msebor@gmail.com>]

On 11/05/2015 10:10 AM, Joseph Myers wrote:
> On Thu, 5 Nov 2015, Martin Sebor wrote:
>
>> optimiziation. We seem to have consensus that even though there
>> might be exceptions (do we know what some of them are?) the vast
>> majority optimizations that have -fxxx options are in fact not
>> performed at -O0. I think reflecting that in the output is more
>> helpful than implying otherwise.
>
> There are several options marked Optimization that are actually matters of
> language semantics that I don't think have anything to do with -O options,
> e.g. -fexceptions and -fcx-limited-range.  The Optimization tag really
> just means that it's saved and restored under certain circumstances.

Both -fexceptions and -fcx-limited-range are reported as disabled
at -O0, so they're not the examples I'm looking for. The set of
options that are reported as disabled by -O0 with the patched GCC
and that, IIUC, are of concern (i.e., that may not need -O to take
effect) are a subset of those below.

Quickly scanning the list, I would guess -frtti, -fshort-enums,
-fsigned-zeros, and -frtapping-math should be in that category,
but there could very well be others.

If it's relatively straightforward to enumerate all such options
then maybe the way to go is to add a bit to struct cl_option to
make it possible to distinguish them from the others in the -O0
-Q --help=optimizers output.

Thanks
Martin

   -faggressive-loop-optimizations 	[disabled by -O0]
   -fasynchronous-unwind-tables 		[disabled by -O0]
   -fauto-inc-dec              		[disabled by -O0]
   -fdce                       		[disabled by -O0]
   -fdelete-null-pointer-checks 		[disabled by -O0]
   -fdse                       		[disabled by -O0]
   -fearly-inlining            		[disabled by -O0]
   -ffunction-cse              		[disabled by -O0]
   -fgcse-lm                   		[disabled by -O0]
   -finline                    		[disabled by -O0]
   -finline-atomics            		[disabled by -O0]
   -fira-hoist-pressure        		[disabled by -O0]
   -fira-share-save-slots      		[disabled by -O0]
   -fira-share-spill-slots     		[disabled by -O0]
   -fivopts                    		[disabled by -O0]
   -fjump-tables               		[disabled by -O0]
   -flifetime-dse              		[disabled by -O0]
   -fmath-errno                		[disabled by -O0]
   -fpeephole                  		[disabled by -O0]
   -fplt                       		[disabled by -O0]
   -fprefetch-loop-arrays      		[disabled by -O0]
   -frename-registers          		[disabled by -O0]
   -frtti                      		[disabled by -O0]
   -fsched-critical-path-heuristic 	[disabled by -O0]
   -fsched-dep-count-heuristic 		[disabled by -O0]
   -fsched-group-heuristic     		[disabled by -O0]
   -fsched-interblock          		[disabled by -O0]
   -fsched-last-insn-heuristic 		[disabled by -O0]
   -fsched-rank-heuristic      		[disabled by -O0]
   -fsched-spec                		[disabled by -O0]
   -fsched-spec-insn-heuristic 		[disabled by -O0]
   -fsched-stalled-insns-dep   		[disabled by -O0]
   -fschedule-fusion           		[disabled by -O0]
   -fshort-enums               		[disabled by -O0]
   -fsigned-zeros              		[disabled by -O0]
   -fsplit-ivs-in-unroller     		[disabled by -O0]
   -fssa-backprop              		[disabled by -O0]
   -fstdarg-opt                		[disabled by -O0]
   -fstrict-volatile-bitfields 		[disabled by -O0]
   -fno-threadsafe-statics     		[disabled by -O0]
   -ftrapping-math             		[disabled by -O0]
   -ftree-cselim               		[disabled by -O0]
   -ftree-forwprop             		[disabled by -O0]
   -ftree-loop-if-convert      		[disabled by -O0]
   -ftree-loop-im              		[disabled by -O0]
   -ftree-loop-ivcanon         		[disabled by -O0]
   -ftree-loop-optimize        		[disabled by -O0]
   -ftree-phiprop              		[disabled by -O0]
   -ftree-reassoc              		[disabled by -O0]
   -ftree-scev-cprop           		[disabled by -O0]
   -fvar-tracking              		[disabled by -O0]
   -fvar-tracking-assignments  		[disabled by -O0]
   -fweb                       		[disabled by -O0]