Travis: Cache toolchain

This should give a significant boost to Travis speeds, because most of the
compile time is spent building the toolchain over and over again.
However, the toolchain (or libc or libm) changes only rarely,
so most rebuilds can skip this step.

The hashing has been put into a separate file to keep it
as decoupled as possible from BuiltIt.sh.

Toolchain/BuildIt.sh

@@ -1,8 +1,10 @@
 #!/usr/bin/env bash
 set -e
-
 # This file will need to be run in bash, for now.
 
+
+# === CONFIGURATION AND SETUP ===
+
 DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 
 echo "$DIR"
@@ -43,6 +45,44 @@ GCC_NAME="gcc-$GCC_VERSION"
 GCC_PKG="${GCC_NAME}.tar.gz"
 GCC_BASE_URL="http://ftp.gnu.org/gnu/gcc"
 
+
+# === CHECK CACHE AND REUSE ===
+
+pushd "$DIR"
+    if [ "${TRY_USE_LOCAL_TOOLCHAIN}" = "y" ] ; then
+        echo "Checking cached toolchain:"
+
+        DEPS_CONFIG="
+            uname=$(uname),TARGET=${TARGET},
+            BuildItHash=$($MD5SUM $(basename $0)),
+            MAKE=${MAKE},MD5SUM=${MD5SUM},NPROC=${NPROC},
+            CC=${CC},CXX=${CXX},with_gmp=${with_gmp},LDFLAGS=${LDFLAGS},
+            BINUTILS_VERSION=${BINUTILS_VERSION},BINUTILS_MD5SUM=${BINUTILS_MD5SUM},
+            GCC_VERSION=${GCC_VERSION},GCC_MD5SUM=${GCC_MD5SUM}"
+        echo "Config is:${DEPS_CONFIG}"
+        if ! DEPS_HASH=$($DIR/ComputeDependenciesHash.sh $MD5SUM <<<"${DEPS_CONFIG}"); then
+            echo "Dependency hashing failed"
+            echo "Will rebuild toolchain from scratch, and NOT SAVE THE RESULT."
+            echo "Someone should look into this, but for now it'll work, albeit inefficient."
+            # Should be empty anyway, but just to make sure:
+            DEPS_HASH=""
+        elif [ -r "Cache/ToolchainLocal_${DEPS_HASH}.tar.gz" ] ; then
+            echo "Cache at Cache/ToolchainLocal_${DEPS_HASH}.tar.gz exists!"
+            echo "Extracting toolchain from cache:"
+            tar xzf "Cache/ToolchainLocal_${DEPS_HASH}.tar.gz"
+            echo "Done 'building' the toolchain."
+            exit 0
+        else
+            echo "Cache at Cache/ToolchainLocal_${DEPS_HASH}.tar.gz does not exist."
+            echo "Will rebuild toolchain from scratch, and save the result."
+        fi
+
+    fi
+popd
+
+
+# === DOWNLOAD AND PATCH ===
+
 pushd "$DIR/Tarballs"
     md5="$($MD5SUM $BINUTILS_PKG | cut -f1 -d' ')"
     echo "bu md5='$md5'"
@@ -98,8 +138,10 @@ pushd "$DIR/Tarballs"
 
 popd
 
-mkdir -p "$PREFIX"
 
+# === COMPILE AND INSTALL ===
+
+mkdir -p "$PREFIX"
 mkdir -p "$DIR/Build/binutils"
 mkdir -p "$DIR/Build/gcc"
 
@@ -162,3 +204,25 @@ pushd "$DIR/Build/"
     popd
 popd
 
+
+# == SAVE TO CACHE ==
+
+pushd "$DIR"
+    if [ "${TRY_USE_LOCAL_TOOLCHAIN}" = "y" ] ; then
+        # TODO: Compress with -z.  It's factor 3, and costs no time.
+        echo "Caching toolchain:"
+
+        if [ -z "${DEPS_HASH}" ] ; then
+            echo "NOT SAVED, because hashing failed."
+            echo "It's computed in the beginning; see there for the error message."
+        elif [ -e "Cache/ToolchainLocal_${DEPS_HASH}.tar.gz" ] ; then
+            # Note: This checks for *existence*.  Initially we checked for
+            # *readability*. If Travis borks permissions, there's not much we can do.
+            echo "Cache exists but was not used?!"
+            echo "Not touching cache then."
+        else
+            mkdir -p Cache/
+            tar czf "Cache/ToolchainLocal_${DEPS_HASH}.tar.gz" Local/
+        fi
+    fi
+popd