Use Default visibility for rustc-generated C symbol declarations

Author: chbaker0

compiler/rustc_codegen_llvm/src/declare.rs

@@ -83,20 +83,16 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         unnamed: llvm::UnnamedAddr,
         fn_type: &'ll Type,
     ) -> &'ll Value {
-        // Declare C ABI functions with the visibility used by C by default.
-        let visibility = if self.tcx.sess.default_hidden_visibility() {
-            llvm::Visibility::Hidden
-        } else {
-            llvm::Visibility::Default
-        };
-
-        declare_raw_fn(self, name, llvm::CCallConv, unnamed, visibility, fn_type)
+        // Declare C ABI functions with Default visibility to allow them to link
+        // dynamically with shared object-provided symbols later on. This is
+        // needed to link intrinsic-generated calls to e.g. libc.so symbols like
+        // memcmp.
+        declare_raw_fn(self, name, llvm::CCallConv, unnamed, llvm::Visibility::Default, fn_type)
     }
 
     /// Declare an entry Function
     ///
-    /// The ABI of this function can change depending on the target (although for now the same as
-    /// `declare_cfn`)
+    /// The ABI of this function can change depending on the target.
     ///
     /// If there’s a value with the same name already declared, the function will
     /// update the declaration and return existing Value instead.

tests/codegen/default-hidden-visibility.rs

@@ -29,3 +29,18 @@ pub static tested_symbol: [u8; 6] = *b"foobar";
 // DEFAULT: @{{.*}}default_hidden_visibility{{.*}}tested_symbol{{.*}} = constant
 // YES:     @{{.*}}default_hidden_visibility{{.*}}tested_symbol{{.*}} = hidden constant
 // NO:      @{{.*}}default_hidden_visibility{{.*}}tested_symbol{{.*}} = constant
+
+pub fn do_memcmp(left: &[u8], right: &[u8]) -> i32 {
+    left.cmp(right) as i32
+}
+
+// CHECK: define {{.*}} @{{.*}}do_memcmp{{.*}} {
+// CHECK: }
+
+// `do_memcmp` should invoke core::intrinsic::compare_bytes which emits a call
+// to the C symbol `memcmp` (at least on x86_64-unknown-linux-gnu). This symbol
+// should *not* be `declare hidden`.
+
+// DEFAULT: declare i32 @memcmp
+// YES:     declare i32 @memcmp
+// NO:      declare i32 @memcmp

tests/ui/intrinsics/default-hidden-visibility-intrinsic.rs

@@ -0,0 +1,10 @@
+//@ build-pass
+//@ compile-flags: -Zdefault-hidden-visibility=yes
+//@ ignore-wasm32 dylibs unsupported, and below does not work
+//@ needs-dynamic-linking
+
+#![crate_type = "dylib"]
+
+pub fn do_memcmp(left: &[u8], right: &[u8]) -> i32 {
+    left.cmp(right) as i32
+}