Auto merge of #96978 - lqd:win_pgo2, r=Mark-Simulacrum

Utilize PGO for windows x64 rustc dist builds

This PR adds PGO support for the CI x64 windows dist builds.

These are the results from running the rustc-perf benchmarks:
![image](https://user-images.githubusercontent.com/247183/177662869-683a8034-7c95-42bf-9900-9ffd66677fcf.png)

Thanks to `@Kobzol,` `@michaelwoerister,` `@wesleywiser,` `@Mark-Simulacrum` for their precious help.

1 files changed, 31 insertions, 2 deletions

diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index c099fedc3a7..3d678b2290d 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -25,6 +25,7 @@ use crate::config::{LlvmLibunwind, TargetSelection};
 use crate::dist;
 use crate::native;
 use crate::tool::SourceType;
+use crate::util::get_clang_cl_resource_dir;
 use crate::util::{exe, is_debug_info, is_dylib, output, symlink_dir, t, up_to_date};
 use crate::LLVM_TOOLS;
 use crate::{CLang, Compiler, DependencyType, GitRepo, Mode};
@@ -772,10 +773,38 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
         if let Some(s) = target_config.and_then(|c| c.llvm_config.as_ref()) {
             cargo.env("CFG_LLVM_ROOT", s);
         }
-        // Some LLVM linker flags (-L and -l) may be needed to link rustc_llvm.
+
+        // Some LLVM linker flags (-L and -l) may be needed to link `rustc_llvm`. Its build script
+        // expects these to be passed via the `LLVM_LINKER_FLAGS` env variable, separated by
+        // whitespace.
+        //
+        // For example:
+        // - on windows, when `clang-cl` is used with instrumentation, we need to manually add
+        // clang's runtime library resource directory so that the profiler runtime library can be
+        // found. This is to avoid the linker errors about undefined references to
+        // `__llvm_profile_instrument_memop` when linking `rustc_driver`.
+        let mut llvm_linker_flags = String::new();
+        if builder.config.llvm_profile_generate && target.contains("msvc") {
+            if let Some(ref clang_cl_path) = builder.config.llvm_clang_cl {
+                // Add clang's runtime library directory to the search path
+                let clang_rt_dir = get_clang_cl_resource_dir(clang_cl_path);
+                llvm_linker_flags.push_str(&format!("-L{}", clang_rt_dir.display()));
+            }
+        }
+
+        // The config can also specify its own llvm linker flags.
         if let Some(ref s) = builder.config.llvm_ldflags {
-            cargo.env("LLVM_LINKER_FLAGS", s);
+            if !llvm_linker_flags.is_empty() {
+                llvm_linker_flags.push_str(" ");
+            }
+            llvm_linker_flags.push_str(s);
+        }
+
+        // Set the linker flags via the env var that `rustc_llvm`'s build script will read.
+        if !llvm_linker_flags.is_empty() {
+            cargo.env("LLVM_LINKER_FLAGS", llvm_linker_flags);
         }
+
         // Building with a static libstdc++ is only supported on linux right now,
         // not for MSVC or macOS
         if builder.config.llvm_static_stdcpp