[COFF] Respect weak externals for mangled symbol searching

We were previously ignoring weak externals during these searches (which
are used for the entry point, exports, and subsystem inference), which
differed from link.exe behavior. It also meant that we could get
different behavior when linking an object file directly vs. packaging it
into a static library, because static library symbol name directories
include weak externals.

Reviewed By: mstorsjo, yozhu

Differential Revision: https://reviews.llvm.org/D139764

Author: smeenai

lld/COFF/SymbolTable.cpp

@@ -809,9 +809,17 @@ std::vector<Symbol *> SymbolTable::getSymsWithPrefix(StringRef prefix) {
 }
 
 Symbol *SymbolTable::findMangle(StringRef name) {
-  if (Symbol *sym = find(name))
-    if (!isa<Undefined>(sym))
+  if (Symbol *sym = find(name)) {
+    if (auto *u = dyn_cast<Undefined>(sym)) {
+      // We're specifically looking for weak aliases that ultimately resolve to
+      // defined symbols, hence the call to getWeakAlias() instead of just using
+      // the weakAlias member variable. This matches link.exe's behavior.
+      if (Symbol *weakAlias = u->getWeakAlias())
+        return weakAlias;
+    } else {
       return sym;
+    }
+  }
 
   // Efficient fuzzy string lookup is impossible with a hash table, so iterate
   // the symbol table once and collect all possibly matching symbols into this

lld/test/COFF/entry-weak-external.s

@@ -0,0 +1,41 @@
+# REQUIRES: x86
+# RUN: rm -rf %t && split-file %s %t
+
+## Ensure that we resolve the entry point to the unmangled weak alias instead of
+## the mangled library definition (which would fail with an undefined symbol).
+
+# RUN: llvm-mc -triple x86_64-windows-msvc -filetype obj -o %t/entry-weak.obj %t/entry-weak.s
+# RUN: llvm-mc -triple x86_64-windows-msvc -filetype obj -o %t/entry-mangled.obj %t/entry-mangled.s
+# RUN: llvm-lib -out:%t/entry-mangled.lib %t/entry-mangled.obj
+# RUN: lld-link -subsystem:console -entry:entry -out:%t/entry-weak-external.exe %t/entry-weak.obj %t/entry-mangled.lib
+# RUN: llvm-readobj --file-headers %t/entry-weak-external.exe | FileCheck %s
+
+## Ensure that we don't resolve the entry point to a weak alias pointing to an
+## undefined symbol (which would have caused the entry point to be 0 instead of
+## an actual address). I can't think of a way of triggering this edge case
+## without using /force:unresolved, which means it likely doesn't matter in
+## practice, but we still match link.exe's behavior for it.
+
+# RUN: llvm-mc -triple x86_64-windows-msvc -filetype obj -o %t/entry-weak-undefined.obj %t/entry-weak-undefined.s
+# RUN: lld-link -subsystem:console -entry:entry -force:unresolved -out:%t/entry-undefined-weak-external.exe \
+# RUN:   %t/entry-weak-undefined.obj %t/entry-mangled.lib
+# RUN: llvm-readobj --file-headers %t/entry-undefined-weak-external.exe | FileCheck %s
+
+# CHECK: AddressOfEntryPoint: 0x1000
+
+#--- entry-weak.s
+.globl default_entry
+default_entry:
+	ret
+
+.weak entry
+entry = default_entry
+
+#--- entry-mangled.s
+.globl "?entry@@YAHXZ"
+"?entry@@YAHXZ":
+	jmp	does_not_exist
+
+#--- entry-weak-undefined.s
+.weak entry
+entry = does_not_exist

lld/test/COFF/subsystem-inference-weak-external.s

@@ -0,0 +1,38 @@
+# REQUIRES: x86
+# RUN: rm -rf %t && split-file %s %t
+
+## Ensure that weak externals are considered during subsystem inference.
+
+# RUN: llvm-mc -triple x86_64-windows-msvc -filetype obj -o %t/cui.obj %t/cui.s
+# RUN: lld-link -out:%t/cui.exe %t/cui.obj
+# RUN: llvm-readobj --file-headers %t/cui.exe | FileCheck --check-prefix=CUI %s
+# RUN: llvm-mc -triple x86_64-windows-msvc -filetype obj -o %t/gui.obj %t/gui.s
+# RUN: lld-link -out:%t/gui.exe %t/gui.obj
+# RUN: llvm-readobj --file-headers %t/gui.exe | FileCheck --check-prefix=GUI %s
+
+# CUI:     Subsystem: IMAGE_SUBSYSTEM_WINDOWS_CUI
+# GUI:     Subsystem: IMAGE_SUBSYSTEM_WINDOWS_GUI
+
+#--- cui.s
+.global default_main
+default_main:
+	ret
+
+.weak main
+main = default_main
+
+.global mainCRTStartup
+mainCRTStartup:
+	ret
+
+#--- gui.s
+.global default_WinMain
+default_WinMain:
+	ret
+
+.weak WinMain
+WinMain = default_WinMain
+
+.global WinMainCRTStartup
+WinMainCRTStartup:
+	ret