Rollup merge of #78566 - JRF63:polly, r=Mark-Simulacrum

Enable LLVM Polly via llvm-args.

I think doing it this way is better than in #51061. Polly has other useful options and we probably don't want to create a `-Z` flag for each one of them.

![results](https://user-images.githubusercontent.com/7283601/97695555-338f7180-1adf-11eb-82bd-5130e0e6fa89.png)

[Benchmark](https://gist.github.com/JRF63/9a6268b91720958e90dbe7abffe20298)

I noticed that `-lto` seems to interfere with polly in this specific microbenchmark, as enabling it causes the perf to drop to that of non-polly builds.

Other related PRs: #75615

config.toml.example

@@ -138,6 +138,9 @@ changelog-seen = 2
 # Whether or not to specify `-DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=YES`
 #allow-old-toolchain = false
 
+# Whether to include the Polly optimizer.
+#polly = false
+
 # =============================================================================
 # General build configuration options
 # =============================================================================

src/bootstrap/config.rs

@@ -99,6 +99,7 @@ pub struct Config {
     pub llvm_version_suffix: Option<String>,
     pub llvm_use_linker: Option<String>,
     pub llvm_allow_old_toolchain: Option<bool>,
+    pub llvm_polly: Option<bool>,
     pub llvm_from_ci: bool,
 
     pub use_lld: bool,
@@ -418,6 +419,7 @@ struct Llvm {
     use_libcxx: Option<bool>,
     use_linker: Option<String>,
     allow_old_toolchain: Option<bool>,
+    polly: Option<bool>,
     download_ci_llvm: Option<StringOrBool>,
 }
 
@@ -762,6 +764,7 @@ impl Config {
             set(&mut config.llvm_use_libcxx, llvm.use_libcxx);
             config.llvm_use_linker = llvm.use_linker.clone();
             config.llvm_allow_old_toolchain = llvm.allow_old_toolchain;
+            config.llvm_polly = llvm.polly;
             config.llvm_from_ci = match llvm.download_ci_llvm {
                 Some(StringOrBool::String(s)) => {
                     assert!(s == "if-available", "unknown option `{}` for download-ci-llvm", s);
@@ -795,6 +798,7 @@ impl Config {
                 check_ci_llvm!(llvm.use_libcxx);
                 check_ci_llvm!(llvm.use_linker);
                 check_ci_llvm!(llvm.allow_old_toolchain);
+                check_ci_llvm!(llvm.polly);
 
                 // CI-built LLVM is shared
                 config.llvm_link_shared = true;

src/bootstrap/native.rs

@@ -257,6 +257,10 @@ impl Step for Llvm {
             enabled_llvm_projects.push("compiler-rt");
         }
 
+        if let Some(true) = builder.config.llvm_polly {
+            enabled_llvm_projects.push("polly");
+        }
+
         // We want libxml to be disabled.
         // See https://github.com/rust-lang/rust/pull/50104
         cfg.define("LLVM_ENABLE_LIBXML2", "OFF");