Auto merge of #88243 - nikic:newpm-2, r=nagisa

Enable new pass manager with LLVM 13

The new pass manager is enabled by default in clang since Clang/LLVM 13. Per the recent discussion on llvm-dev (https://lists.llvm.org/pipermail/llvm-dev/2021-August/152305.html) the legacy pass manager will be unmaintained in LLVM 14 and removed entirely in LLVM 15.

This switches us to use the new pass manager if LLVM >= 13 is used. It's possible to still use the old pass manager using `-Z new-llvm-pass-manager=no`.

2 files changed, 7 insertions, 3 deletions

diff --git a/compiler/rustc_codegen_llvm/src/back/write.rs b/compiler/rustc_codegen_llvm/src/back/write.rs
index 791604a1827..92199f611ba 100644
--- a/compiler/rustc_codegen_llvm/src/back/write.rs
+++ b/compiler/rustc_codegen_llvm/src/back/write.rs
@@ -370,8 +370,9 @@ fn get_pgo_use_path(config: &ModuleConfig) -> Option<CString> {
 }
 
 pub(crate) fn should_use_new_llvm_pass_manager(config: &ModuleConfig) -> bool {
-    // The new pass manager is disabled by default.
-    config.new_llvm_pass_manager.unwrap_or(false)
+    // The new pass manager is enabled by default for LLVM >= 13.
+    // This matches Clang, which also enables it since Clang 13.
+    config.new_llvm_pass_manager.unwrap_or_else(|| llvm_util::get_version() >= (13, 0, 0))
 }
 
 pub(crate) unsafe fn optimize_with_new_llvm_pass_manager(
diff --git a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
index 1a0cfd58885..48eb50953a9 100644
--- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
@@ -1004,7 +1004,10 @@ LLVMRustOptimizeWithNewPassManager(
 #endif
   bool NeedThinLTOBufferPasses = UseThinLTOBuffers;
   if (!NoPrepopulatePasses) {
-    if (OptLevel == OptimizationLevel::O0) {
+    // The pre-link pipelines don't support O0 and require using budilO0DefaultPipeline() instead.
+    // At the same time, the LTO pipelines do support O0 and using them is required.
+    bool IsLTO = OptStage == LLVMRustOptStage::ThinLTO || OptStage == LLVMRustOptStage::FatLTO;
+    if (OptLevel == OptimizationLevel::O0 && !IsLTO) {
 #if LLVM_VERSION_GE(12, 0)
       for (const auto &C : PipelineStartEPCallbacks)
         PB.registerPipelineStartEPCallback(C);