From ffb10db6fa080e4c225e16928aec052d4c0baa95 Mon Sep 17 00:00:00 2001
From: Keith Packard <keithp@keithp.com>
Date: Fri, 2 Sep 2022 23:07:05 -0700
Subject: [PATCH] Allow default libc to be specified to configure

The default C library is normally computed based on the target
triplet. However, for embedded systems, it can be useful to leave the
triplet alone while changing which C library is used by default. Other
C libraries may still be available on the system so the compiler and
can be used by specifying suitable include and library paths at build
time.

If an unknown --with-default-libc= value is provided, emit an error
and stop.

Signed-off-by: Keith Packard <keithp@keithp.com>
---
 gcc/config.gcc   |   48 ++++++++++++++++++++++++++++++++++++++++--------
 gcc/configure.ac |    4 ++++
 2 files changed, 44 insertions(+), 8 deletions(-)

--- a/gcc/config.gcc
+++ b/gcc/config.gcc
@@ -695,6 +695,8 @@
 # Common C libraries.
 tm_defines="$tm_defines LIBC_GLIBC=1 LIBC_UCLIBC=2 LIBC_BIONIC=3 LIBC_MUSL=4"
 
+default_libc=""
+
 # 32-bit x86 processors supported by --with-arch=.  Each processor
 # MUST be separated by exactly one space.
 x86_archs="athlon athlon-4 athlon-fx athlon-mp athlon-tbird \
@@ -911,16 +913,16 @@
   esac
   case $target in
     *-*-*android*)
-      tm_defines="$tm_defines DEFAULT_LIBC=LIBC_BIONIC"
+      default_libc=LIBC_BIONIC
       ;;
     *-*-*uclibc* | *-*-uclinuxfdpiceabi)
-      tm_defines="$tm_defines DEFAULT_LIBC=LIBC_UCLIBC"
+      default_libc=LIBC_UCLIBC
       ;;
     *-*-*musl*)
-      tm_defines="$tm_defines DEFAULT_LIBC=LIBC_MUSL"
+      default_libc=LIBC_MUSL
       ;;
     *)
-      tm_defines="$tm_defines DEFAULT_LIBC=LIBC_GLIBC"
+      default_libc=LIBC_GLIBC
       ;;
   esac
   # Assume that glibc or uClibc or Bionic are being used and so __cxa_atexit
@@ -1035,7 +1037,8 @@
   case ${enable_threads} in
     "" | yes | posix) thread_file='posix' ;;
   esac
-  tm_defines="$tm_defines DEFAULT_LIBC=LIBC_UCLIBC SINGLE_LIBC"
+  tm_defines="$tm_defines SINGLE_LIBC"
+  default_libc=LIBC_UCLIBC
   ;;
 *-*-rdos*)
   use_gcc_stdint=wrap
@@ -1721,13 +1724,13 @@
 
 		case ${target} in
 		    csky-*-linux-gnu*)
-			tm_defines="$tm_defines DEFAULT_LIBC=LIBC_GLIBC"
+			default_libc=LIBC_GLIBC
 			# Force .init_array support.  The configure script cannot always
 			# automatically detect that GAS supports it, yet we require it.
 			gcc_cv_initfini_array=yes
 			;;
 		    csky-*-linux-uclibc*)
-			tm_defines="$tm_defines DEFAULT_LIBC=LIBC_UCLIBC"
+			default_libc=LIBC_UCLIBC
 			default_use_cxa_atexit=no
 			;;
 		    *)
@@ -3125,7 +3128,7 @@
 	tmake_file="${tmake_file} t-linux rs6000/t-linux64 rs6000/t-fprules rs6000/t-ppccomm"
 	tmake_file="${tmake_file} rs6000/t-vxworks"
 
-	tm_defines="$tm_defines DEFAULT_LIBC=LIBC_GLIBC"
+	default_libc=LIBC_GLIBC
 	extra_objs="$extra_objs linux.o rs6000-linux.o"
 	;;
 powerpc-wrs-vxworks*)
@@ -6079,3 +6082,32 @@
 	fi
 	;;
 esac
+
+case "${with_default_libc}" in
+glibc)
+    default_libc=LIBC_GLIBC
+    ;;
+uclibc)
+    default_libc=LIBC_UCLIBC
+    ;;
+bionic)
+    default_libc=LIBC_BIONIC
+    ;;
+musl)
+    default_libc=LIBC_MUSL
+    ;;
+"")
+    ;;
+*)
+    echo "Unknown libc in --with-default-libc=$with_default_libc" 1>&2
+    exit 1
+    ;;
+esac
+
+case "$default_libc" in
+"")
+        ;;
+*)
+	tm_defines="$tm_defines DEFAULT_LIBC=$default_libc"
+	;;
+esac
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -2546,6 +2546,10 @@
 fi
 AC_SUBST(inhibit_libc)
 
+AC_ARG_WITH(default-libc,
+	[AS_HELP_STRING([--with-default-libc],
+			[Use specified default C library])])
+
 # When building gcc with a cross-compiler, we need to adjust things so
 # that the generator programs are still built with the native compiler.
 # Also, we cannot run fixincludes.