shims/super/cc: tweak optimisation flag handling for runtime CPU detection builds

[carlocab]

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---

We need to allow -mcpu and -mtune flags to avoid breaking builds on
arm64 macOS. See, for example, Homebrew/homebrew-core#212051.

Also, let's reinstate adding our own optimisation flags if the compiler
was not invoked with a conflicting optimisation flag.

[Copilot]

Pull Request Overview

This pull request refactors the handling of optimization flags to support runtime CPU detection builds on arm64 macOS by adjusting how flags such as -mcpu and -mtune are processed.

• Removed -mtune and -mcpu flag matching from the first case and placed them in a separate condition for CPU detection.
• Replaced a blanket concatenation of optflags with an iteration that checks for conflicts with existing flags.

Comments suppressed due to low confidence (1)

Library/Homebrew/shims/super/cc:204

• Confirm that moving the -mtune and -mcpu flags here maintains the intended flag precedence and does not affect builds on platforms other than arm64.

when /^-march=.+/, /^-mtune=.+/, /^-mcpu=.+/

[Bo98]

We need to allow -mcpu and -mtune flags to avoid breaking builds on
arm64 macOS. See, for example, Homebrew/homebrew-core#212051.

Why? nss does not pass -mcpu or -mtune

[carlocab]

Why? nss does not pass -mcpu or -mtune

Not really sure, but dropping the -mcpu/-mtune filtering fixes this failure: https://github.com/Homebrew/homebrew-core/actions/runs/13962241797/job/39085667674#step:3:1173

[Bo98]

Does it? Looks like it's actually our stripping of -march that actually made it work in the first place.

It's compiling as aarch32 because this is wrong: https://github.com/nss-dev/nss/blob/b6a1be96fba1c400d77698bde71ccaec6796e86d/coreconf/Darwin.mk#L18 + https://github.com/nss-dev/nss/blob/b6a1be96fba1c400d77698bde71ccaec6796e86d/coreconf/Darwin.mk#L34.

You can override CPU_ARCH in the formula fortunately to aarch64

[carlocab]

You can override CPU_ARCH in the formula fortunately to aarch64

Won't that make the build assume a PowerPC build?

https://github.com/nss-dev/nss/blob/b6a1be96fba1c400d77698bde71ccaec6796e86d/coreconf/Darwin.mk#L35-L38

[carlocab]

Dropped the -mtune and -mcpu part of the flags for now.

[Bo98]

Won't that make the build assume a PowerPC build?

Ah right yeah, probably needs something like:

diff --git a/coreconf/Darwin.mk b/coreconf/Darwin.mk
index 0569e1819..d1893adb4 100644
--- a/coreconf/Darwin.mk
+++ b/coreconf/Darwin.mk
@@ -31,13 +31,19 @@ override CPU_ARCH	= x86
 endif
 else
 ifeq (arm,$(CPU_ARCH))
-# Nothing set for arm currently.
+ifdef USE_64
+CC              += -arch arm64
+CCC             += -arch arm64
+override CPU_ARCH	= aarch64
+endif
 else
+ifeq (ppc,$(CPU_ARCH))
 OS_REL_CFLAGS	= -Dppc
 CC              += -arch ppc
 CCC             += -arch ppc
 endif
 endif
+endif
 
 ifneq (,$(MACOS_SDK_DIR))
     GCC_VERSION_FULL := $(shell $(CC) -dumpversion)

or I guess inreplaceing the arm away also works as a quick fix since the -arch flag really doesn't matter

[michaelsproul]

Hi @carlocab, apologies if this is the wrong place to post.

I'm debugging a formula build failure in this PR:

• lighthouse 7.0.0 homebrew-core#220922

I think I've tracked the failure down to the cc shim's handling of the -mno-avx flag, and noticed that you recently made changes to the shim, and therefore must understand how it works 😁

Is it possible the shim would filter out the -mno-avx flag during the "flag detection" compile that the Rust build system runs – leading Rust's cc crate to think that the -mno-avx flag is supported when it actually is not?

Again, sorry if I'm barking up the wrong tree. I'm not very experienced in Homebrew's workings, nor Ruby.