Re: Add -falign-all-functions

[Richard Biener <rguenther@suse.de>]

On Wed, 17 Jan 2024, Jan Hubicka wrote:

> > On Wed, 17 Jan 2024, Jan Hubicka wrote:
> > 
> > > > 
> > > > I meant the new option might be named -fmin-function-alignment=
> > > > rather than -falign-all-functions because of how it should
> > > > override all other options.
> > > 
> > > I was also pondering about both names.  -falign-all-functions has the
> > > advantage that it is similar to all the other alignment flags that are
> > > all called -falign-XXX
> > > 
> > > but both options are finte for me.
> > > > 
> > > > Otherwise is there an updated patch to look at?
> > > 
> > > I will prepare one.  So shall I drop the max-skip support for alignment
> > > and rename the flag?
> > 
> > Yes.
> OK, here is updated version.
> Bootstrapped/regtested on x86_64-linux, OK?
> 
> gcc/ChangeLog:
> 
> 	* common.opt (flimit-function-alignment): Reorder so file is
> 	alphabetically ordered.
> 	(flimit-function-alignment): New flag.

fmin-function-alignment

OK with that change.

Thanks,
Richard.

> 	* doc/invoke.texi (-fmin-function-alignment): Document
> 	(-falign-jumps,-falign-labels): Document that this is an optimization
> 	bypassed in cold code.
> 	* varasm.cc (assemble_start_function): Honor -fmin-function-alignment.
> 
> diff --git a/gcc/common.opt b/gcc/common.opt
> index 5f0a101bccb..6e85853f086 100644
> --- a/gcc/common.opt
> +++ b/gcc/common.opt
> @@ -1040,9 +1040,6 @@ Align the start of functions.
>  falign-functions=
>  Common RejectNegative Joined Var(str_align_functions) Optimization
>  
> -flimit-function-alignment
> -Common Var(flag_limit_function_alignment) Optimization Init(0)
> -
>  falign-jumps
>  Common Var(flag_align_jumps) Optimization
>  Align labels which are only reached by jumping.
> @@ -2277,6 +2274,10 @@ fmessage-length=
>  Common RejectNegative Joined UInteger
>  -fmessage-length=<number>	Limit diagnostics to <number> characters per line.  0 suppresses line-wrapping.
>  
> +fmin-function-alignment=
> +Common Joined RejectNegative UInteger Var(flag_min_function_alignment) Optimization
> +Align the start of every function.
> +
>  fmodulo-sched
>  Common Var(flag_modulo_sched) Optimization
>  Perform SMS based modulo scheduling before the first scheduling pass.
> @@ -2601,6 +2602,9 @@ starts and when the destructor finishes.
>  flifetime-dse=
>  Common Joined RejectNegative UInteger Var(flag_lifetime_dse) Optimization IntegerRange(0, 2)
>  
> +flimit-function-alignment
> +Common Var(flag_limit_function_alignment) Optimization Init(0)
> +
>  flive-patching
>  Common RejectNegative Alias(flive-patching=,inline-clone) Optimization
>  
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 43fd3c3a3cd..456374d9446 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -546,6 +546,7 @@ Objective-C and Objective-C++ Dialects}.
>  -falign-jumps[=@var{n}[:@var{m}:[@var{n2}[:@var{m2}]]]]
>  -falign-labels[=@var{n}[:@var{m}:[@var{n2}[:@var{m2}]]]]
>  -falign-loops[=@var{n}[:@var{m}:[@var{n2}[:@var{m2}]]]]
> +-fmin-function-alignment=[@var{n}]
>  -fno-allocation-dce -fallow-store-data-races
>  -fassociative-math  -fauto-profile  -fauto-profile[=@var{path}]
>  -fauto-inc-dec  -fbranch-probabilities
> @@ -14177,6 +14178,9 @@ Align the start of functions to the next power-of-two greater than or
>  equal to @var{n}, skipping up to @var{m}-1 bytes.  This ensures that at
>  least the first @var{m} bytes of the function can be fetched by the CPU
>  without crossing an @var{n}-byte alignment boundary.
> +This is an optimization of code performance and alignment is ignored for
> +functions considered cold.  If alignment is required for all functions,
> +use @option{-fmin-function-alignment}.
>  
>  If @var{m} is not specified, it defaults to @var{n}.
>  
> @@ -14240,6 +14244,8 @@ Enabled at levels @option{-O2}, @option{-O3}.
>  Align loops to a power-of-two boundary.  If the loops are executed
>  many times, this makes up for any execution of the dummy padding
>  instructions.
> +This is an optimization of code performance and alignment is ignored for
> +loops considered cold.
>  
>  If @option{-falign-labels} is greater than this value, then its value
>  is used instead.
> @@ -14262,6 +14268,8 @@ Enabled at levels @option{-O2}, @option{-O3}.
>  Align branch targets to a power-of-two boundary, for branch targets
>  where the targets can only be reached by jumping.  In this case,
>  no dummy operations need be executed.
> +This is an optimization of code performance and alignment is ignored for
> +jumps considered cold.
>  
>  If @option{-falign-labels} is greater than this value, then its value
>  is used instead.
> @@ -14275,6 +14283,14 @@ The maximum allowed @var{n} option value is 65536.
>  
>  Enabled at levels @option{-O2}, @option{-O3}.
>  
> +@opindex fmin-function-alignment=@var{n}
> +@item -fmin-function-alignment
> +Specify minimal alignment of functions to the next power-of-two greater than or
> +equal to @var{n}. Unlike @option{-falign-functions} this alignment is applied
> +also to all functions (even those considered cold).  The alignment is also not
> +affected by @option{-flimit-function-alignment}
> +
> +
>  @opindex fno-allocation-dce
>  @item -fno-allocation-dce
>  Do not remove unused C++ allocations in dead code elimination.
> @@ -14371,7 +14387,7 @@ To use the link-time optimizer, @option{-flto} and optimization
>  options should be specified at compile time and during the final link.
>  It is recommended that you compile all the files participating in the
>  same link with the same options and also specify those options at
> -link time.  
> +link time.
>  For example:
>  
>  @smallexample
> diff --git a/gcc/varasm.cc b/gcc/varasm.cc
> index d2c879b7da4..ccf97a5a496 100644
> --- a/gcc/varasm.cc
> +++ b/gcc/varasm.cc
> @@ -1939,11 +1939,16 @@ assemble_start_function (tree decl, const char *fnname)
>  
>    /* Tell assembler to move to target machine's alignment for functions.  */
>    align = floor_log2 (align / BITS_PER_UNIT);
> +  /* Handle forced alignment.  This really ought to apply to all functions,
> +     since it is used by patchable entries.  */
> +  if (flag_min_function_alignment && align < flag_min_function_alignment)
> +    align = flag_min_function_alignment;
> +
>    if (align > 0)
>      {
>        ASM_OUTPUT_ALIGN (asm_out_file, align);
>      }
>  
>    /* Handle a user-specified function alignment.
>       Note that we still need to align to DECL_ALIGN, as above,
>       because ASM_OUTPUT_MAX_SKIP_ALIGN might not do any alignment at all.  */
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)