From 71a31b30d91c02e0fffe86843481c9fecd6c8fce Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Thu, 5 Mar 2026 14:46:07 +0000
Subject: [PATCH] Always use the ThinLTO pipeline for pre-link optimizations

When using cargo this was already effectively done for all dependencies
as cargo passes -Clinker-plugin-lto without -Clto=fat/thin.
-Clinker-plugin-lto assumes that ThinLTO will be used. The ThinLTO
pre-link pipeline is faster than the fat LTO one. And according to the
benchmarks in [1] there is barely any runtime performance difference
between executables that used fat LTO with the fat vs ThinLTO pre-link
pipeline.

[1]: https://discourse.llvm.org/t/rfc-a-unified-lto-bitcode-frontend/61774
---
 compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
index 568335f7dcb51..f42624700b596 100644
--- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
@@ -840,11 +840,8 @@ extern "C" LLVMRustResult LLVMRustOptimize(
         }
         break;
       case LLVMRustOptStage::PreLinkThinLTO:
-        MPM = PB.buildThinLTOPreLinkDefaultPipeline(OptLevel);
-        NeedThinLTOBufferPasses = false;
-        break;
       case LLVMRustOptStage::PreLinkFatLTO:
-        MPM = PB.buildLTOPreLinkDefaultPipeline(OptLevel);
+        MPM = PB.buildThinLTOPreLinkDefaultPipeline(OptLevel);
         NeedThinLTOBufferPasses = false;
         break;
       case LLVMRustOptStage::ThinLTO: