[AArch64][GlobalISel] Fall back if disabling neon/fp in the translator.

[aemerson]

Summary

The previous technique relied on early-exiting the legalizer predicate initialization, leaving an empty rule table. That causes a fallback for most instructions, but some have legacy rules defined like G_ZEXT which can try continue, but then crash.

We should fall back earlier, in the translator, to avoid this issue.

[paquette]

LGTM

llvm/include/llvm/CodeGen/GlobalISel/CallLowering.h

411 ↗

(On Diff #331056)

Would it make sense for this hook to just only take in the MF, and then try to call getFunction or whatever to (try to) get at the Function?

[aemerson]

llvm/include/llvm/CodeGen/GlobalISel/CallLowering.h

411 ↗

(On Diff #331056)

Yeah, will do.

[aemerson]

Didn't mean to update this diff, ignore.

llvm/lib/CodeGen/GlobalISel/Utils.cpp

bool llvm::isTriviallyDead(const MachineInstr &MI,
                            const MachineRegisterInfo &MRI) {
   // FIXME: This logical is mostly duplicated with
   // DeadMachineInstructionElim::isDead. Why is LOCAL_ESCAPE not considered in
   // MachineInstr::isLabel?
 
   // Don't delete frame allocation labels.
   if (MI.getOpcode() == TargetOpcode::LOCAL_ESCAPE)
     return false;
+  // LIFETIME markers should be preserved even if they seem dead.
+  if (MI.getOpcode() == TargetOpcode::LIFETIME_START ||
+      MI.getOpcode() == TargetOpcode::LIFETIME_END)
+    return false;
 
   // If we can move an instruction, we can remove it.  Otherwise, it has
   // a side-effect of some sort.
   bool SawStore = false;
   if (!MI.isSafeToMove(/*AA=*/nullptr, SawStore) && !MI.isPHI())
     return false;
 
   // Instructions without side-effects are dead iff they only define dead vregs.

llvm/test/CodeGen/AArch64/GlobalISel/lifetime-marker-no-dce.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64 -run-pass=aarch64-prelegalizer-combiner -global-isel -verify-machineinstrs %s -o - | FileCheck %s
+# Check that we don't DCE the lifetime markers even though they don't have any users.
+---
+name:            test_lifetime_no_dce
+alignment:       4
+tracksRegLiveness: true
+frameInfo:
+  maxAlignment:    4
+stack:
+  - { id: 0, size: 4, alignment: 4 }
+machineFunctionInfo: {}
+body:             |
+  bb.1:
+    ;%0:_(p0) = G_FRAME_INDEX %stack.0.slot
+    ; CHECK-LABEL: name: test_lifetime_no_dce
+    ; CHECK: LIFETIME_START %stack.0
+    ; CHECK: LIFETIME_END %stack.0
+    ; CHECK: RET_ReallyLR
+    LIFETIME_START %stack.0
+    LIFETIME_END %stack.0
+    RET_ReallyLR
+
+...