Re: [PATCH] Support multilib-aware target lib flags self-specs overriding

[Alexandre Oliva <oliva@adacore.com>]

On Jun  3, 2022, Alexandre Oliva <oliva@adacore.com> wrote:

> On Jun  1, 2022, Hans-Peter Nilsson <hp@bitrange.com> wrote:

>> -fmultiflags?

> That works for me.  I favored -multiflags slightly, because the intended
> use is for it to stand for other -m flags, but --multiflags AKA
> -fmultiflags will do as well.


> Now, is there interest in this feature?  (As in, does it any make sense
> for me to post a revised patch, or should I keep it downstream?)

FTR, here's the patch I ended up with, adjusted to use -fmultiflags.


Support multilib-aware target lib flags self-specs overriding

This patch introduces -fmultiflags, short for multilib TFLAGS, as an
option that does nothing by default, but that can be added to TFLAGS
and mapped to useful options by driver self-specs.

Regstrapped on x86_64-linux-gnu.  Posted mainly FTR, but...  I wouldn't
mind putting it in, so...  Ok to install?  ;-)


for  gcc/ChangeLog

	* common.opt (fmultiflags): New.
	* doc/invoke.texi: Document it.
	* gcc.cc (driver_self_specs): Discard it.
	* opts.cc (common_handle_option): Ignore it in the driver.
---
 gcc/common.opt      |    4 ++++
 gcc/doc/invoke.texi |   30 +++++++++++++++++++++++++++++-
 gcc/gcc.cc          |    6 +++++-
 gcc/opts.cc         |    4 ++++
 4 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/gcc/common.opt b/gcc/common.opt
index e7a51e882bade..814966c4562fe 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2177,6 +2177,10 @@ fmove-loop-stores
 Common Var(flag_move_loop_stores) Optimization
 Move stores out of loops.
 
+fmultiflags
+Common Driver
+Building block for specs-based multilib-aware TFLAGS.
+
 fdce
 Common Var(flag_dce) Init(1) Optimization
 Use the RTL dead code elimination pass.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index bde59ff047249..f9c53df37b28b 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -725,7 +725,7 @@ Objective-C and Objective-C++ Dialects}.
 -flto-report  -flto-report-wpa  -fmem-report-wpa @gol
 -fmem-report  -fpre-ipa-mem-report  -fpost-ipa-mem-report @gol
 -fopt-info  -fopt-info-@var{options}@r{[}=@var{file}@r{]} @gol
--fprofile-report @gol
+-fmultiflags  -fprofile-report @gol
 -frandom-seed=@var{string}  -fsched-verbose=@var{n} @gol
 -fsel-sched-verbose  -fsel-sched-dump-cfg  -fsel-sched-pipelining-verbose @gol
 -fstats  -fstack-usage  -ftime-report  -ftime-report-details @gol
@@ -18927,6 +18927,34 @@ allocation for the WPA phase only.
 Makes the compiler print some statistics about permanent memory
 allocation before or after interprocedural optimization.
 
+@item -fmultiflags
+@opindex fmultiflags
+This option enables multilib-aware @code{TFLAGS} to be used to build
+target libraries with options different from those the compiler is
+configured to use by default, through the use of specs (@xref{Spec
+Files}) set up by compiler internals, by the target, or by builders at
+configure time.
+
+Like @code{TFLAGS}, this allows the target libraries to be built for
+portable baseline environments, while the compiler defaults to more
+demanding ones.  That's useful because users can easily override the
+defaults the compiler is configured to use to build their own programs,
+if the defaults are not ideal for their target environment, whereas
+rebuilding the runtime libraries is usually not as easy or desirable.
+
+Unlike @code{TFLAGS}, the use of specs enables different flags to be
+selected for different multilibs.  The way to accomplish that is to
+build with @samp{make TFLAGS=-fmultiflags}, after configuring
+@samp{--with-specs=%@{fmultiflags:...@}}.
+
+This option is discarded by the driver once it's done processing driver
+self spec.
+
+It is also useful to check that @code{TFLAGS} are being used to build
+all target libraries, by configuring a non-bootstrap compiler
+@samp{--with-specs='%@{!fmultiflags:%emissing TFLAGS@}'} and building
+the compiler and target libraries.
+
 @item -fprofile-report
 @opindex fprofile-report
 Makes the compiler print some statistics about consistency of the
diff --git a/gcc/gcc.cc b/gcc/gcc.cc
index 5cbb38560b266..24d3544e68140 100644
--- a/gcc/gcc.cc
+++ b/gcc/gcc.cc
@@ -1347,7 +1347,11 @@ static const char *const multilib_defaults_raw[] = MULTILIB_DEFAULTS;
 
 static const char *const driver_self_specs[] = {
   "%{fdump-final-insns:-fdump-final-insns=.} %<fdump-final-insns",
-  DRIVER_SELF_SPECS, CONFIGURE_SPECS, GOMP_SELF_SPECS, GTM_SELF_SPECS
+  DRIVER_SELF_SPECS, CONFIGURE_SPECS, GOMP_SELF_SPECS, GTM_SELF_SPECS,
+  /* This discards -fmultiflags at the end of self specs processing in the
+     driver, so that it is effectively Ignored, without actually marking it as
+     Ignored, which would get it discarded before self specs could remap it.  */
+  "%<fmultiflags"
 };
 
 #ifndef OPTION_DEFAULT_SPECS
diff --git a/gcc/opts.cc b/gcc/opts.cc
index fe0293e4283df..1ac9554a341c6 100644
--- a/gcc/opts.cc
+++ b/gcc/opts.cc
@@ -3257,6 +3257,10 @@ common_handle_option (struct gcc_options *opts,
       dc->report_bug = value;
       break;
 
+    case OPT_fmultiflags:
+      gcc_checking_assert (lang_mask == CL_DRIVER);
+      break;
+
     default:
       /* If the flag was handled in a standard way, assume the lack of
 	 processing here is intentional.  */


-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts.  Ask me about <https://stallmansupport.org>