LTO: Add API to choose whether to embed uselists

Reverse libLTO's default behaviour for preserving use-list order in
bitcode, and add API for controlling it.  The default setting is now
`false` (don't preserve them), which is consistent with `clang`'s
default behaviour.

Users of libLTO should call `lto_codegen_should_embed_uselists(CG,true)`
prior to calling `lto_codegen_write_merged_modules()` whenever the
output file isn't part of the production workflow in order to reproduce
results with subsequent calls to `llc`.

(I haven't added tests since `llvm-lto` (the test tool for LTO) doesn't
support bitcode output, and even if it did: there isn't actually a good
way to test whether a tool has passed the flag.  If the order is already
"natural" (if the order will already round-trip) then no use-list
directives are emitted at all.  At some point I'll circle back to add
tests to `llvm-as` (etc.) that they actually respect the flag, at which
point I can somehow add a test here as well.)

llvm-svn: 235943

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/tools/lto/lto.cpp b/llvm/tools/lto/lto.cpp
index e66a7786a14..d6ceebed728 100644
--- a/llvm/tools/lto/lto.cpp
+++ b/llvm/tools/lto/lto.cpp
@@ -338,3 +338,8 @@ void lto_codegen_set_should_internalize(lto_code_gen_t cg,
                                         bool ShouldInternalize) {
   unwrap(cg)->setShouldInternalize(ShouldInternalize);
 }
+
+void lto_codegen_set_should_embed_uselists(lto_code_gen_t cg,
+                                           lto_bool_t ShouldEmbedUselists) {
+  unwrap(cg)->setShouldEmbedUselists(ShouldEmbedUselists);
+}