Add a function to check whether the C compiler is using LTO.
To determine this, we compile a dummy source unit. In the GCC case,
we check whether the resulting object file contains ".gnu.lto*"
sections. In the clang case, we check whether a valid LLVM bytecode
file was output rather than a regular object.

The goal of this change is to reduce the amount of USE=lto abuse,
and have a consistent cross-package way of enabling LTO via setting
appropriate CFLAGS and CXXFLAGS.

Signed-off-by: Michał Górny <mgorny@gentoo.org>
---
eclass/toolchain-funcs.eclass | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)

diff --git a/eclass/toolchain-funcs.eclass b/eclass/toolchain-funcs.eclass index 5da93063866b..cde84e6f34c8 100644
--- a/eclass/toolchain-funcs.eclass
+++ b/eclass/toolchain-funcs.eclass
@@ -1230,4 +1230,25 @@ tc-get-build-ptr-size() {
die "Could not determine CBUILD pointer size"
}

+# @FUNCTION: tc-is-lto
+# @RETURN: Shell true if we are using LTO, shell false otherwise
+tc-is-lto() {
+ local f="${T}/test-lto.o"
+
+ case $(tc-get-compiler-type) in
+ clang)
+ $(tc-getCC) ${CFLAGS} -c -o "${f}" -x c - <<<"" || die + # If LTO is used, clang will output bytecode and llvm-bcanalyzer
+ # will run successfully. Otherwise, it will output plain object
+ # file and llvm-bcanalyzer will exit with error.
+ llvm-bcanalyzer "${f}" &>/dev/null && return 0
+ ;;
+ gcc)
+ $(tc-getCC) ${CFLAGS} -c -o "${f}" -x c - <<<"" || die + [[ $($(tc-getREADELF) -S "${f}") == *.gnu.lto* ]] &&