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Let cc-rs choose the correct optimization flags for MSVC (/O2 instead of /Ox) #1596

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Oct 13, 2023
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Let cc-rs choose the correct optimization flags for MSVC (/O2 instead of /Ox) #1596

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11 changes: 3 additions & 8 deletions build.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -613,14 +613,9 @@ fn cc(file: &Path, ext: &str, target: &Target, include_dir: &Path, out_file: &Pa
let _ = c.define("NDEBUG", None);
}

if compiler.is_like_msvc() {
if std::env::var("OPT_LEVEL").unwrap() == "0" {
let _ = c.flag("/Od"); // Disable optimization for debug builds.
// run-time checking: (s)tack frame, (u)ninitialized variables
let _ = c.flag("/RTCsu");
} else {
let _ = c.flag("/Ox"); // Enable full optimization.
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Do you agree that cc-rs does the right thing automatically, so that we can just remove both this line and the analogous "/Od" line above? I would prefer to do that, if cc-rs is automatically doing the right thing. (In general, I want to remove as much logic from build.rs that duplicates that cc-rs does as possible; see recent PRs that change build.rs to this effect.)

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I haven't previously used cc-rs so I assumed manually specifying optimization flags was somehow necessary 😅
Well, cc-rs can determine the correct flag for each compiler: https://github.com/rust-lang/cc-rs/blob/2447a2ba5f455c00b1563193e125b60eebbd8ebe/src/lib.rs#L1748-L1749

I quickly checked the docs and there's this function that lets you manually specify the optimization level, but it also says that the OPT_LEVEL environment variable is used to determine the correct value otherwise (which is what is done manually here).

About "/Od", it is the default so no need to specify it: https://learn.microsoft.com/en-us/cpp/build/reference/od-disable-debug?view=msvc-170

It's not clear to me whether "/RTCsu" is on by default:
https://learn.microsoft.com/en-us/cpp/build/reference/rtc-run-time-error-checks?view=msvc-170

The Visual Studio IDE does enable it for debug builds but that might not be true when using the compiler directly. We might need to read a little more, or just keep it to be sure.

tl;dr
Yes, I totally agree, no optimization flags should be manually set unless there's a good reason for doing it. The "/RTCsu" situation is not 100% clear to me

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I took another look, but I couldn't conclusively find anything indicating that /RTCsu is enabled by default. Therefore I left that part but removed the rest. If you think you don't need /RTCsu, then the whole if statement can be removed, otherwise I would say the change is ready to be merged

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@andoalon I would be happy to have the /RTCsu removed.

}
if compiler.is_like_msvc() && std::env::var("OPT_LEVEL").unwrap() == "0" {
// run-time checking: (s)tack frame, (u)ninitialized variables
let _ = c.flag("/RTCsu");
}

// Allow cross-compiling without a target sysroot for these targets.
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