[lld-macho]Limit "cannot-export-hidden-symbol" warnings to only 3 to avoid crowding logs.

Details:
We often use wildcard symbols in the exported_symbols list, and sometimes they match autohide symbols, which triggers these "cannot export hidden symbols" warnings that can be a bit noisy.
It'd be more user-friendly if LLD could truncate these.

Differential Revision: https://reviews.llvm.org/D159095

Author: oontvoo

lld/MachO/Driver.cpp

@@ -1355,8 +1355,10 @@ static void createAliases() {
 }
 
 static void handleExplicitExports() {
+  static constexpr int kMaxWarnings = 3;
   if (config->hasExplicitExports) {
-    parallelForEach(symtab->getSymbols(), [](Symbol *sym) {
+    std::atomic<uint64_t> warningsCount{0};
+    parallelForEach(symtab->getSymbols(), [&warningsCount](Symbol *sym) {
       if (auto *defined = dyn_cast<Defined>(sym)) {
         if (config->exportedSymbols.match(sym->getName())) {
           if (defined->privateExtern) {
@@ -1367,8 +1369,12 @@ static void handleExplicitExports() {
               // The former can be exported but the latter cannot.
               defined->privateExtern = false;
             } else {
-              warn("cannot export hidden symbol " + toString(*defined) +
-                   "\n>>> defined in " + toString(defined->getFile()));
+              // Only print the first 3 warnings verbosely, and
+              // shorten the rest to avoid crowding logs.
+              if (warningsCount.fetch_add(1, std::memory_order_relaxed) <
+                  kMaxWarnings)
+                warn("cannot export hidden symbol " + toString(*defined) +
+                     "\n>>> defined in " + toString(defined->getFile()));
             }
           }
         } else {
@@ -1378,6 +1384,9 @@ static void handleExplicitExports() {
         dysym->shouldReexport = config->exportedSymbols.match(sym->getName());
       }
     });
+    if (warningsCount > kMaxWarnings)
+      warn("<... " + Twine(warningsCount - kMaxWarnings) +
+           " more similar warnings...>");
   } else if (!config->unexportedSymbols.empty()) {
     parallelForEach(symtab->getSymbols(), [](Symbol *sym) {
       if (auto *defined = dyn_cast<Defined>(sym))

lld/test/MachO/export-options.s

@@ -4,6 +4,7 @@
 # RUN: echo "" | llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/empty.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t/default.s -o %t/default.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t/lazydef.s -o %t/lazydef.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t/too-many-warnings.s -o %t/too-many-warnings.o
 # RUN: llvm-ar --format=darwin rcs %t/lazydef.a %t/lazydef.o
 
 ## Check that mixing exported and unexported symbol options yields an error
@@ -209,6 +210,17 @@
 # RUN: llvm-objdump --macho --exports-trie %t/unexport-dylib | FileCheck %s \
 # RUN:   --check-prefix=EMPTY-TRIE
 
+## Check that warnings are truncated to the first 3 only.
+# RUN: %no-fatal-warnings-lld -dylib %t/too-many-warnings.o -o %t/too-many.out \
+# RUN:         -exported_symbol "_private_extern*" 2>&1 | \
+# RUN:     FileCheck --check-prefix=TRUNCATE %s
+
+# TRUNCATE: warning: cannot export hidden symbol _private_extern{{.+}}
+# TRUNCATE: warning: cannot export hidden symbol _private_extern{{.+}}
+# TRUNCATE: warning: cannot export hidden symbol _private_extern{{.+}}
+# TRUNCATE: warning: <... 7 more similar warnings...>
+# TRUNCATE-EMPTY:
+
 #--- default.s
 
 .globl _keep_globl, _hide_globl, _tlv
@@ -283,3 +295,45 @@ _foo:
 .private_extern _foo
 _foo:
   retq
+
+#--- too-many-warnings.s
+.private_extern _private_extern1
+.private_extern _private_extern2
+.private_extern _private_extern3
+.private_extern _private_extern4
+.private_extern _private_extern5
+.private_extern _private_extern6
+.private_extern _private_extern7
+.private_extern _private_extern8
+.private_extern _private_extern9
+.private_extern _private_extern10
+
+_private_extern1:
+  retq
+
+_private_extern2:
+  retq
+
+_private_extern3:
+  retq
+
+_private_extern4:
+  retq
+
+_private_extern5:
+  retq
+
+_private_extern6:
+  retq
+
+_private_extern7:
+  retq
+
+_private_extern8:
+  retq
+
+_private_extern9:
+  retq
+
+_private_extern10:
+  retq