[clang][WebAssembly] Support the -m(no-)red-zone flag.

[alexrp]

LLVM's WebAssembly backend supports the noredzone function attribute, but this support was not exposed in Clang. This just mirrors what is done for AArch64, PowerPC, and x86, minus the Apple-specific checks that are irrelevant here.

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-driver

Author: Alex Rønne Petersen (alexrp)

Changes

LLVM's WebAssembly backend supports the noredzone function attribute, but this support was not exposed in Clang.

Note: I don't have commit access.

---

Full diff: https://github.com/llvm/llvm-project/pull/119997.diff

1 Files Affected:

• (modified) clang/lib/Driver/ToolChains/Clang.cpp (+3)

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 7ef55a33547c50..21117cd71b651e 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -2404,6 +2404,9 @@ void Clang::AddWebAssemblyTargetArgs(const ArgList &Args,
   if (!Args.hasArg(options::OPT_fvisibility_EQ,
                    options::OPT_fvisibility_ms_compat))
     CmdArgs.push_back("-fvisibility=hidden");
+
+  if (!Args.hasFlag(options::OPT_mred_zone, options::OPT_mno_red_zone, true))
+    CmdArgs.push_back("-disable-red-zone");
 }
 
 void Clang::AddVETargetArgs(const ArgList &Args, ArgStringList &CmdArgs) const {

[alexrp]

@sunfishcode would you mind reviewing this one? (I've pinged a few times here but I'm not sure any of the relevant WASM folks are actually seeing it due to the labels...)

[sunfishcode]

Is there a use case that needs to disable the red zone globally on Wasm? My understanding is that -mno-red-zone exists for compatibility reasons that don't apply to Wasm. I'm hesitant to add flags adding implicit ABI variants in the absence of use cases. At least with the function attribute it's explicit in the code.

[alexrp]

I don't have any particular use case in mind. I was just going over the targets for which we allow -m(no-)red-zone in Zig and noticed that WASM is a strange outlier in that the LLVM backend supports the option but Clang doesn't expose it. This means that we have to distinguish between whether the option is supported in Zig code (where we go straight to LLVM) vs C/C++ code (zig cc, where we go through the Clang driver). We can do that if we must, but it just seemed easier to expose the functionality that's already there in the backend anyway.

[sunfishcode]

Makes sense. My instinct here is to wait until we have a concrete use case.