[RAFast] Don't dereference MBB::end

When RAFast sees liveins in on a basic block, it uses that information
to initialize the availability of the registers. The called
method uses an instruction as one of its argument and in the liveins
case, RAFast was dereferencing MBB::begin which can be MBB::end for
empty basic block.

Change the API of definePhysReg to use MachineBasicBlock::iterator
instead of MachineInstr so that we don't dereference an
invalid iterator while making the call.

rdar://problem/36952401

llvm-svn: 323710

1 files changed, 26 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/AArch64/fast-regalloc-empty-bb-with-liveins.mir b/llvm/test/CodeGen/AArch64/fast-regalloc-empty-bb-with-liveins.mir
new file mode 100644
index 00000000000..fc19173a176
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/fast-regalloc-empty-bb-with-liveins.mir
@@ -0,0 +1,26 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64-apple-ios -run-pass regallocfast -o - %s | FileCheck %s
+# This test used to crash the fast register alloc.
+# Basically, when a basic block has liveins, the fast regalloc
+# was deferencing the begin iterator of this block. However,
+# when this block is empty and it will just crashed!
+---
+name:            crashing
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: crashing
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK:   liveins: %x0, %x1
+  ; CHECK: bb.1:
+  ; CHECK:   renamable %w0 = MOVi32imm -1
+  ; CHECK:   RET_ReallyLR implicit killed %w0
+  bb.1:
+    liveins: %x0, %x1
+
+  bb.2:
+    %0:gpr32 = MOVi32imm -1
+    %w0 = COPY %0
+    RET_ReallyLR implicit %w0
+
+...