[LLD][COFF] Allow overriding EC alias symbols with alternate names

[cjacek]

No description provided.

[llvmbot]

@llvm/pr-subscribers-llvm-ir
@llvm/pr-subscribers-platform-windows

@llvm/pr-subscribers-lld-coff

Author: Jacek Caban (cjacek)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/113456.diff

2 Files Affected:

• (modified) lld/COFF/Driver.cpp (+3-2)
• (added) lld/test/COFF/arm64ec-altnames.s (+53)

diff --git a/lld/COFF/Driver.cpp b/lld/COFF/Driver.cpp
index e7f768789271fa..f9c78482e8aa79 100644
--- a/lld/COFF/Driver.cpp
+++ b/lld/COFF/Driver.cpp
@@ -2518,9 +2518,10 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
         Symbol *sym = ctx.symtab.find(from);
         if (!sym)
           continue;
-        if (auto *u = dyn_cast<Undefined>(sym))
-          if (!u->weakAlias)
+        if (auto *u = dyn_cast<Undefined>(sym)) {
+          if (!u->weakAlias || u->isECAlias(ctx.config.machine))
             u->setWeakAlias(ctx.symtab.addUndefined(to));
+        }
       }
 
       // If any inputs are bitcode files, the LTO code generator may create
diff --git a/lld/test/COFF/arm64ec-altnames.s b/lld/test/COFF/arm64ec-altnames.s
new file mode 100644
index 00000000000000..a346ced7f5b7ad
--- /dev/null
+++ b/lld/test/COFF/arm64ec-altnames.s
@@ -0,0 +1,53 @@
+# REQUIRES: aarch64
+
+# RUN: llvm-mc -filetype=obj -triple=arm64ec-windows %s -o %t.obj
+# RUN: llvm-mc -filetype=obj -triple=arm64ec-windows %S/Inputs/loadconfig-arm64ec.s -o %t-loadconfig.obj
+
+# RUN: lld-link -machine:arm64ec -dll -noentry -out:%t1.dll %t.obj %t-loadconfig.obj "-alternatename:#func=altsym"
+
+# RUN: llvm-objdump -d %t1.dll | FileCheck --check-prefix=DISASM %s
+# DISASM:      0000000180001000 <.text>:
+# DISASM-NEXT: 180001000: 52800020     mov     w0, #0x1                // =1
+# DISASM-NEXT: 180001004: d65f03c0     ret
+# DISASM-NOT: .thnk
+
+# RUN: llvm-readobj --hex-dump=.test %t1.dll | FileCheck --check-prefix=TESTSEC %s
+# TESTSEC: 0x180004000 00100000 00100000
+
+# RUN: lld-link -machine:arm64ec -dll -noentry -out:%t2.dll %t.obj %t-loadconfig.obj -alternatename:func=altsym
+
+# RUN: llvm-objdump -d %t2.dll | FileCheck --check-prefix=DISASM2 %s
+# DISASM2:      Disassembly of section .text:
+# DISASM2-EMPTY:
+# DISASM2-NEXT: 0000000180001000 <.text>:
+# DISASM2-NEXT: 180001000: 52800020     mov     w0, #0x1                // =1
+# DISASM2-NEXT: 180001004: d65f03c0     ret
+# DISASM2-EMPTY:
+# DISASM2-NEXT: Disassembly of section .thnk:
+# DISASM2-EMPTY:
+# DISASM2-NEXT: 0000000180005000 <.thnk>:
+# DISASM2-NEXT: 180005000: 52800040     mov     w0, #0x2                // =2
+# DISASM2-NEXT: 180005004: d65f03c0     ret
+
+# RUN: llvm-readobj --hex-dump=.test %t2.dll | FileCheck --check-prefix=TESTSEC2 %s
+# TESTSEC2: 0x180004000 00100000 00500000
+
+        .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

[cjacek]

Similar to the handling of lazy symbols, EC aliases behave like undefined symbols in this context.

[cjacek]

I just noticed that we may need special-casing func->#func aliases. It needs more testing, I changed it to a draft for now.

[cjacek]

More experimentation with MSVC revealed that -alternatename is effective on anti-dependency symbols if either the aliased symbol is undefined or the symbol is mangled. In practice, this distinction helps differentiate between an alias to the guest exit thunk (which is always mangled) and a demangled alias to the implementation symbol. I kept it compatible; it’s now ready for review.

[cjacek]

Fixed a typo in test.

[mstorsjo]

LGTM