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.


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.

4 files changed, 42 insertions, 2 deletions

diff --git a/gcc/common.opt b/gcc/common.opt
index bce3e51..26e9d1c 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2174,6 +2174,10 @@ 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 e9207a3..5077cd7 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -738,7 +738,7 @@ 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
@@ -19344,6 +19344,34 @@ 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 98a239c..830ab88 100644
--- a/gcc/gcc.cc
+++ b/gcc/gcc.cc
@@ -1327,7 +1327,11 @@ 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 ae079fc..3797784 100644
--- a/gcc/opts.cc
+++ b/gcc/opts.cc
@@ -3248,6 +3248,10 @@ 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.  */