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-C lto -O does not enable LLVM optimizations for the LTO passes #22525
Description
-O seems to function like an alias for -C opt-level=2, but -O does not enable certain LTO LLVM optimizations (inlining?), whereas -C opt-level=2 does. This seems like a mistake.
The problem is this code in librustc_trans/back/lto.rs:
let opt = sess.opts.cg.opt_level.unwrap_or(0) as libc::c_uint;
let builder = llvm::LLVMPassManagerBuilderCreate();
llvm::LLVMPassManagerBuilderSetOptLevel(builder, opt);
llvm::LLVMPassManagerBuilderPopulateLTOPassManager(builder, pm,
/* Internalize = */ False,
/* RunInliner = */ True);
llvm::LLVMPassManagerBuilderDispose(builder);
sess.opts.cg.opt_level is the -C opt-level setting, which is None with -O.
test.rs:
pub fn test() {}
output:
$ rustc test.rs --crate-type staticlib --emit llvm-ir -C lto -O
$ ls -l test.ll
-rw-rw-r-- 1 rprichard rprichard 36920440 Feb 19 00:25 test.ll
$ rustc test.rs --crate-type staticlib --emit llvm-ir -C lto -C opt-level=2
$ ls -l test.ll
-rw-rw-r-- 1 rprichard rprichard 1312070 Feb 19 00:25 test.ll
I think we should base the LLVM optimization level on sess.opts.optimize. (Maybe it ought to use write::get_llvm_opt_level?)
Activity
dotdash commentedon Feb 19, 2015
Nice find! Probably not noticed by many because those using
-C ltoalso go straight to-C opt-level=3as well. Do you want to make a PR to fix this?rprichard commentedon Feb 19, 2015
I'd be happy to make a PR, though I probably won't get to it until tomorrow.
Use sess.opts.optimize instead of sess.opts.cg.opt_level for LTO optl…
Merge pull request rust-lang#22525 from Veykril/push-wympnqnklmvy