Re: Add -falign-all-functions

[Jan Hubicka <hubicka@ucw.cz>]

> 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.
	* 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.  */