We end up calling resolveBranchVA(), which asserts for Undefineds.
As fix, just return early in Writer::run() if there are any diagnostics
after processing relocations (which is where undefined symbol errors are
emitted). This matches what the ELF port does.
Differential Revision: https://reviews.llvm.org/D109079
if (config->entry && !isa<Undefined>(config->entry))
prepareBranchTarget(config->entry);
scanRelocations();
+
+ // Do not proceed if there was an undefined symbol.
+ if (errorCount())
+ return;
+
if (in.stubHelper->isNeeded())
in.stubHelper->setup();
scanSymbols();
createOutputSections<LP>();
+
// After this point, we create no new segments; HOWEVER, we might
// yet create branch-range extension thunks for architectures whose
// hardware call instructions have limited range, e.g., ARM(64).
--- /dev/null
+# REQUIRES: aarch64
+
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t.o
+## This shouldn't assert.
+# RUN: not %lld -arch arm64 -lSystem -o %t/thunk %t.o 2>&1 | FileCheck %s
+
+# CHECK: error: undefined symbol: _g
+
+.subsections_via_symbols
+
+.p2align 2
+
+.globl _main, _g
+
+.globl _main
+_main:
+ bl _g
+ ret
+
+_filler1:
+.space 0x4000000
+
+_filler2:
+.space 0x4000000