[WebAssembly] Enable -Werror=strict-prototypes by default

[sunfish]

Summary

WebAssembly, the architecture, is not designed to support unprototyped calling conventions. For example, clang tends to assume that it can use varargs to implement parts of the unprototyped function rules, which works on most other architectures, however WebAssembly has no straightforward way of allowing functions to be callable as both varargs and non-varargs. LLVM's backend does find ways to make things work in some cases, however the support is incomplete. To protect users from accidentally trying to compile such code and encountering bugs, this patch enables -Werror=strict-prototypes by default for the WebAssembly target.

[sbc100]

Might want to line wrap your change description.

[sbc100]

Hmm.. actually this is probably going to break the waterfall pretty badly. But maybe we can fix it by re-disabling this explicitly when we build the gcc tests.

[efriedma]

If someone is compiling C code that doesn't have undefined behavior, it should work; if it doesn't, that's a clear bug. (As far as I know, there shouldn't be any issues here, but if there are, file a bug and CC me.)

WebAssembly is not the only platform where varargs and non-varargs calls are incompatible. Some other popular platforms where this is a problem are 64-bit Windows and hard-float AArch32. So I don't think it makes sense to enable this specifically for one target.

[sunfish]

With https://reviews.llvm.org/D48471 and related work, it's notnow less urgent to do this. And since there were objections to having target-specific warnings anyway, let's close this.

lib/Driver/ToolChains/Clang.cpp

if (Value.getAsInteger(10, Mregparm)) {
             << A->getOption().getName() << Value;
       }
     }
   }
 }
 
 void Clang::AddWebAssemblyTargetArgs(const ArgList &Args,
                                      ArgStringList &CmdArgs) const {
+  // WebAssembly doesn't fully support code without prototypes.
+  CmdArgs.push_back("-Werror=strict-prototypes");
+
   // Default to "hidden" visibility.
   if (!Args.hasArg(options::OPT_fvisibility_EQ,
                    options::OPT_fvisibility_ms_compat)) {
     CmdArgs.push_back("-fvisibility");
     CmdArgs.push_back("hidden");
   }
 }
 

test/Driver/wasm-toolchain.c

 // A basic clang -cc1 command-line. WebAssembly is somewhat special in
-// enabling -fvisibility=hidden by default.
+// enabling -fvisibility=hidden and -Werror-strict-prototypes by default.
 
 // RUN: %clang %s -### -no-canonical-prefixes -target wasm32-unknown-unknown 2>&1 | FileCheck -check-prefix=CC1 %s
-// CC1: clang{{.*}} "-cc1" "-triple" "wasm32-unknown-unknown" {{.*}} "-fvisibility" "hidden" {{.*}}
+// CC1: clang{{.*}} "-cc1" "-triple" "wasm32-unknown-unknown" {{.*}} "-Werror=strict-prototypes" "-fvisibility" "hidden" {{.*}}
 
 // Ditto, but ensure that a user -fvisibility=default disables the default
 // -fvisibility=hidden.
 
 // RUN: %clang %s -### -target wasm32-unknown-unknown -fvisibility=default 2>&1 | FileCheck -check-prefix=FVISIBILITY_DEFAULT %s
 // FVISIBILITY_DEFAULT-NOT: hidden
 
+// Ensure that a user -Wno-strict-prototypes disabled the default
+// -Werror=strict-prototypes.
+
+// RUN: %clang %s -### -target wasm32-unknown-unknown -Wno-strict-prototypes 2>&1 | FileCheck -check-prefix=WNO_STRICT_PROTOTYPES %s
+// WNO_STRICT_PROTOTYPES: "-Werror=strict-prototypes" {{.*}} "-Wno-strict-prototypes"
+
 // A basic C link command-line.
 
 // RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 | FileCheck -check-prefix=LINK %s
 // LINK: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK: lld{{.*}}" "-flavor" "wasm" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C link command-line with optimization.
 
 // RUN: %clang -### -O2 -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 | FileCheck -check-prefix=LINK_OPT %s
 // LINK_OPT: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK_OPT: lld{{.*}}" "-flavor" "wasm" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"