[LLD][COFF] Avoid resolving symbols with -alternatename if the target is undefined

This change fixes an issue with the use of -alternatename in the MSVC CRT, where both mangled
and demangled symbol names are specified. Without this patch, the demangled name could be resolved
to an anti-dependency alias of the target. Since chaining anti-dependency aliases is not allowed,
this results in an undefined symbol.

The root cause isn't specific to ARM64EC, it can affect other targets as well, even when
anti-dependency aliases aren't involved. The accompanying test case demonstrates a scenario where
the symbol could be resolved from an archive. However, because the archive member is pulled in
after the first pass of alternate name resolution, and archive members don't override weak aliases,
eager resolution would incorrectly skip it.

Author: cjacek

lld/COFF/SymbolTable.cpp

@@ -1364,7 +1364,19 @@ void SymbolTable::resolveAlternateNames() {
             !isArm64ECMangledFunctionName(u->getName()))
           continue;
       }
-      u->setWeakAlias(addUndefined(to));
+
+      // Check if the destination symbol is defined. If not, skip it.
+      // It may still be resolved later if more input files are added.
+      // Also skip anti-dependency targets, as they can't be chained anyway.
+      Symbol *toSym = find(to);
+      if (!toSym)
+        continue;
+      auto toUndef = dyn_cast<Undefined>(toSym);
+      if (toUndef && (!toUndef->weakAlias || toUndef->isAntiDep))
+        continue;
+      if (toSym->isLazy())
+        forceLazy(toSym);
+      u->setWeakAlias(toSym);
     }
   }
 }

lld/test/COFF/alternatename-alias.s

@@ -0,0 +1,15 @@
+// REQUIRES: x86
+
+// Check that a weak alias can be used as an alternate name target.
+// RUN: llvm-mc -filetype=obj -triple=x86_64-windows %s -o %t.obj
+// RUN: lld-link -dll -noentry %t.obj -alternatename:sym=altsym
+
+        .data
+        .rva sym
+
+        .weak altsym
+        .set altsym,a
+
+        .globl a
+a:
+        .word 1

lld/test/COFF/alternatename-antidep.s

@@ -0,0 +1,16 @@
+// REQUIRES: x86
+
+// Check that an anti-dependency alias can't be used as an alternate name target.
+// RUN: llvm-mc -filetype=obj -triple=x86_64-windows %s -o %t.obj
+// RUN: not lld-link -dll -noentry %t.obj -alternatename:sym=altsym 2>&1 | FileCheck %s
+// CHECK: error: undefined symbol: sym
+
+        .data
+        .rva sym
+
+        .weak_anti_dep altsym
+        .set altsym,a
+
+        .globl a
+a:
+        .word 1

lld/test/COFF/alternatename-lib.s

@@ -0,0 +1,43 @@
+// REQUIRES: x86
+// RUN: split-file %s %t.dir && cd %t.dir
+
+// RUN: llvm-mc -filetype=obj -triple=x86_64-windows refab.s -o refab.obj
+// RUN: llvm-mc -filetype=obj -triple=x86_64-windows aa.s -o aa.obj
+// RUN: llvm-mc -filetype=obj -triple=x86_64-windows b.s -o b.obj
+// RUN: llvm-mc -filetype=obj -triple=x86_64-windows antidep.s -o antidep.obj
+// RUN: llvm-lib -out:aa.lib aa.obj
+// RUN: llvm-lib -out:b.lib b.obj
+
+// Check that -alternatename with an undefined target does not prevent the symbol from being resolved to a library,
+// once another alternate name is resolved and pulls in the source symbol.
+// RUN: lld-link -out:out.dll -dll -noentry -machine:amd64 refab.obj aa.lib -alternatename:a=aa -alternatename:b=undef
+
+// Check that -alternatename with an anti-dependency target does not prevent the symbol from being resolved to a library,
+// after another alternate name is resolved and pulls in the source symbol.
+// RUN: lld-link -out:out2.dll -dll -noentry -machine:amd64 antidep.obj refab.obj aa.lib -alternatename:a=aa -alternatename:b=u
+
+#--- refab.s
+        .data
+        .rva a
+        .rva b
+
+#--- aa.s
+        .globl aa
+aa:
+        .word 1
+
+        .section .drectve, "yn"
+        .ascii "/defaultlib:b.lib"
+
+#--- b.s
+        .globl b
+b:
+        .word 2
+
+#--- antidep.s
+        .weak_anti_dep u
+        .set u,d
+
+        .globl d
+d:
+        .word 3

lld/test/COFF/arm64ec-altnames.s

@@ -2,6 +2,7 @@ REQUIRES: aarch64
 RUN: split-file %s %t.dir && cd %t.dir
 
 RUN: llvm-mc -filetype=obj -triple=arm64ec-windows ext.s -o ext.obj
+RUN: llvm-mc -filetype=obj -triple=arm64ec-windows ext-mangled.s -o ext-mangled.obj
 RUN: llvm-mc -filetype=obj -triple=arm64ec-windows impl.s -o impl.obj
 RUN: llvm-mc -filetype=obj -triple=arm64ec-windows impl-cpp.s -o impl-cpp.obj
 RUN: llvm-mc -filetype=obj -triple=arm64ec-windows %S/Inputs/loadconfig-arm64ec.s -o loadconfig.obj
@@ -49,6 +50,20 @@ RUN: lld-link -machine:arm64ec -dll -noentry -out:out4.dll impl-cpp.obj loadconf
 RUN: llvm-objdump -d out4.dll | FileCheck --check-prefix=DISASM %s
 RUN: llvm-readobj --hex-dump=.test out4.dll | FileCheck --check-prefix=TESTSEC %s
 
+# Check that when both mangled and demangled alternate names are used,
+# only the one whose target is defined is used (the mangled one in this case).
+
+RUN: lld-link -machine:arm64ec -dll -noentry -out:out5.dll ext-mangled.obj loadconfig.obj "-alternatename:#func=#altsym" -alternatename:func=altsym
+RUN: llvm-objdump -d out5.dll | FileCheck --check-prefix=DISASM %s
+RUN: llvm-readobj --hex-dump=.test out5.dll | FileCheck --check-prefix=TESTSEC %s
+
+# Check that when both mangled and demangled alternate names are used,
+# only the one whose target is defined is used (the demangled one in this case).
+
+RUN: lld-link -machine:arm64ec -dll -noentry -out:out6.dll ext.obj loadconfig.obj "-alternatename:#func=#altsym" -alternatename:func=altsym
+RUN: llvm-objdump -d out6.dll | FileCheck --check-prefix=DISASM2 %s
+RUN: llvm-readobj --hex-dump=.test out6.dll | FileCheck --check-prefix=TESTSEC2 %s
+
 #--- ext.s
         .weak_anti_dep func
 .set func, "#func"
@@ -70,6 +85,30 @@ altsym:
         mov w0, #1
         ret
 
+#--- ext-mangled.s
+        .weak_anti_dep func
+.set func, "#func"
+        .weak_anti_dep "#func"
+.set "#func", thunksym
+
+        .section .test, "r"
+        .rva func
+        .rva "#func"
+
+        .section .thnk,"xr",discard,thunksym
+thunksym:
+        mov w0, #2
+        ret
+
+        .section .text,"xr",discard,"#altsym"
+        .globl "#altsym"
+"#altsym":
+        mov w0, #1
+        ret
+
+        .weak_anti_dep altsym
+        .set altsym,"#altsym"
+
 #--- impl.s
         .weak_anti_dep func
 .set func, "#func"