jitlayers: Use GlobalISel on AArch64 at -O0/-O1

GlobalISel is LLVM's modern instruction selector that is designed to replace
both FastISel and SelectionDAG. On AArch64, it is mature and enabled by default
at -O0 in upstream LLVM.

This enables GlobalISel with fallback mode on AArch64, which provides faster
instruction selection than SelectionDAG while maintaining correctness by
falling back to SelectionDAG for unsupported patterns.

Note: This requires RemoveJuliaAddrspacesPass to run before codegen, which is
already the case in the current pipeline (see pipeline.cpp comment about
GlobalISel not liking Julia's address spaces).

Co-authored-by: Claude <[email protected]>

Author: IanButterworth

src/jitlayers.cpp

@@ -1897,7 +1897,17 @@ void optimizeDLSyms(Module &M) JL_NOTSAFEPOINT_LEAVE JL_NOTSAFEPOINT_ENTER {
 void fixupTM(TargetMachine &TM) {
     auto TheTriple = TM.getTargetTriple();
     if (jl_options.opt_level < 2) {
-        if (!TheTriple.isARM() && !TheTriple.isPPC64())
+        // Try GlobalISel on AArch64 - it's the default in LLVM at -O0 and
+        // is apparently generally faster than SelectionDAG while producing good code.
+        // Use fallback mode so unsupported patterns fall back to SelectionDAG.
+        // Note: Requires RemoveJuliaAddrspacesPass to run before codegen
+        // because GlobalISel doesn't handle Julia's custom address spaces.
+        if (TheTriple.isAArch64()) {
+            TM.setGlobalISel(true);
+            TM.setGlobalISelAbort(GlobalISelAbortMode::Disable);
+            TM.setFastISel(false);
+        }
+        else if (!TheTriple.isARM() && !TheTriple.isPPC64()) {
             TM.setFastISel(true);
         else    // FastISel seems to be buggy Ref #13321
             TM.setFastISel(false);