Don't force -O1 with ThinLTO

This doesn't seem to be necessary anymore, although I don't know
at which point or why that changed.

Forcing -O1 makes some tests fail under NewPM, because NewPM also
performs inlining at -O1, so it ends up performing much more
optimization in practice than before.

Author: nikic

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -584,12 +584,6 @@ pub(crate) fn run_pass_manager(
         if write::should_use_new_llvm_pass_manager(config) {
             let opt_stage = if thin { llvm::OptStage::ThinLTO } else { llvm::OptStage::FatLTO };
             let opt_level = config.opt_level.unwrap_or(config::OptLevel::No);
-            // See comment below for why this is necessary.
-            let opt_level = if let config::OptLevel::No = opt_level {
-                config::OptLevel::Less
-            } else {
-                opt_level
-            };
             write::optimize_with_new_llvm_pass_manager(cgcx, module, config, opt_level, opt_stage);
             debug!("lto done");
             return;
@@ -603,26 +597,10 @@ pub(crate) fn run_pass_manager(
             llvm::LLVMRustAddPass(pm, pass.unwrap());
         }
 
-        // When optimizing for LTO we don't actually pass in `-O0`, but we force
-        // it to always happen at least with `-O1`.
-        //
-        // With ThinLTO we mess around a lot with symbol visibility in a way
-        // that will actually cause linking failures if we optimize at O0 which
-        // notable is lacking in dead code elimination. To ensure we at least
-        // get some optimizations and correctly link we forcibly switch to `-O1`
-        // to get dead code elimination.
-        //
-        // Note that in general this shouldn't matter too much as you typically
-        // only turn on ThinLTO when you're compiling with optimizations
-        // otherwise.
         let opt_level = config
             .opt_level
             .map(|x| to_llvm_opt_settings(x).0)
             .unwrap_or(llvm::CodeGenOptLevel::None);
-        let opt_level = match opt_level {
-            llvm::CodeGenOptLevel::None => llvm::CodeGenOptLevel::Less,
-            level => level,
-        };
         with_llvm_pmb(module.module_llvm.llmod(), config, opt_level, false, &mut |b| {
             if thin {
                 llvm::LLVMRustPassManagerBuilderPopulateThinLTOPassManager(b, pm);