bootstrap: build compiler-builtins with -Z emit-stack-sizes

Author: japaric

src/bootstrap/bin/rustc.rs

@@ -181,6 +181,30 @@ fn main() {
  cmd.arg("-C").arg(format!("debug-assertions={}", debug_assertions));
  }
 
+ // Build `compiler_builtins` with `-Z emit-stack-sizes` to add stack usage information.
+ //
+ // When you use this flag with Cargo you get stack usage information on all crates compiled
+ // from source, and when you are using LTO you also get information on pre-compiled crates
+ // like `core` and `std`. However, there's an exception: `compiler_builtins`. This crate
+ // is special and doesn't participate in LTO because it's always linked as a separate object
+ // file. Due to this it's impossible to get information about this crate using `RUSTFLAGS`
+ // + Cargo, or `cargo rustc`.
+ //
+ // To make the stack usage information of this crate available to Cargo based stack usage
+ // analysis tools we compile `compiler_builtins` with the `-Z emit-stack-sizes` flag. The
+ // flag is known to currently work with targets that produce ELF files so we limit the use
+ // of the flag to those targets.
+ //
+ // NOTE(japaric) if this ever causes problem with an LLVM upgrade or any PR feel free to
+ // remove it or comment it out
+ if crate_name == "compiler_builtins"
+ && (target.contains("-linux-")
+ || target.contains("-none-eabi")
+ || target.ends_with("-none-elf"))
+ {
+ cmd.arg("-Z").arg("emit-stack-sizes");
+ }
+
  if let Ok(s) = env::var("RUSTC_CODEGEN_UNITS") {
  cmd.arg("-C").arg(format!("codegen-units={}", s));
  }