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Auto merge of #71267 - pnkfelix:issue-71248-dont-need-exports-all-green, r=nagisa

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attempt to recover perf by removing `exports_all_green`

attempt to recover perf by removing `exports_all_green` flag.

cc #71248

(My hypothesis is that my use of this flag was an overly conservative generalization of PR #67020.)
This commit is contained in:
bors 2020-04-21 04:35:17 +00:00
commit 25f070d1ed
1 changed files with 22 additions and 10 deletions
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32
src/librustc_codegen_llvm/back/lto.rs
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@ -500,15 +500,32 @@ fn thin_lto(
let module_name = module_name_to_str(module_name);
// If (1.) the module hasn't changed, and (2.) none of the modules
// it imports from nor exports to have changed, *and* (3.) the
// import and export sets themselves have not changed from the
// previous compile when it was last ThinLTO'ed, then we can re-use
// the post-ThinLTO version of the module. Otherwise, freshly
// perform LTO optimization.
// it imports from have changed, *and* (3.) the import and export
// sets themselves have not changed from the previous compile when
// it was last ThinLTO'ed, then we can re-use the post-ThinLTO
// version of the module. Otherwise, freshly perform LTO
// optimization.
//
// (Note that globally, the export set is just the inverse of the
// import set.)
//
// For further justification of why the above is necessary and sufficient,
// see the LLVM blog post on ThinLTO:
//
// http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html
//
// which states the following:
//
// ```quote
// any particular ThinLTO backend must be redone iff:
//
// 1. The corresponding (primary) module’s bitcode changed
// 2. The list of imports into or exports from the module changed
// 3. The bitcode for any module being imported from has changed
// 4. Any global analysis result affecting either the primary module
// or anything it imports has changed.
// ```
//
// This strategy means we can always save the computed imports as
// canon: when we reuse the post-ThinLTO version, condition (3.)
// ensures that the current import set is the same as the previous
@ -531,13 +548,8 @@ fn thin_lto(
let imports_all_green = curr_imports
.iter()
.all(|imported_module| green_modules.contains_key(imported_module));
let exports_all_green = curr_exports
.iter()
.all(|exported_module| green_modules.contains_key(exported_module));
if imports_all_green
&& equivalent_as_sets(prev_imports, curr_imports)
&& exports_all_green
&& equivalent_as_sets(prev_exports, curr_exports)
{
let work_product = green_modules[module_name].clone();