[LivePhysRegs] Preserve pristine regs in blocks with no successors.

One common source of blocks with no successors is calls to noreturn
functions; we want to preserve pristine registers in case they throw an
exception.

The whole pristine register thing is messy (we should really prefer to
explicitly model registers), but this fills a hole in the model for now.

Fixes https://bugs.llvm.org/show_bug.cgi?id=36073.

Differential Revision: https://reviews.llvm.org/D42509



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323559 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Eli Friedman

Diff for: lib/CodeGen/LivePhysRegs.cpp

@@ -225,10 +225,10 @@ void LivePhysRegs::addLiveOutsNoPristines(const MachineBasicBlock &MBB) {
 
 void LivePhysRegs::addLiveOuts(const MachineBasicBlock &MBB) {
   const MachineFunction &MF = *MBB.getParent();
-  if (!MBB.succ_empty()) {
+  if (!MBB.isReturnBlock()) {
     addPristines(MF);
     addLiveOutsNoPristines(MBB);
-  } else if (MBB.isReturnBlock()) {
+  } else {
     // For the return block: Add all callee saved registers.
     const MachineFrameInfo &MFI = MF.getFrameInfo();
     if (MFI.isCalleeSavedInfoValid())

Diff for: test/CodeGen/Thumb/stm-scavenging.ll

@@ -0,0 +1,47 @@
+; RUN: llc < %s | FileCheck %s
+target triple = "thumbv6---gnueabi"
+
+; Use STM to save the three registers
+; CHECK-LABEL: use_stm:
+; CHECK: .save   {r7, lr}
+; CHECK: .setfp  r7, sp
+; CHECK: stm r3!, {r0, r1, r2}
+; CHECK: bl throws_1
+define void @use_stm(i32 %a, i32 %b, i32 %c, i32* %d) local_unnamed_addr noreturn "no-frame-pointer-elim"="true" {
+entry:
+  %arrayidx = getelementptr inbounds i32, i32* %d, i32 2
+  store i32 %a, i32* %arrayidx, align 4
+  %arrayidx1 = getelementptr inbounds i32, i32* %d, i32 3
+  store i32 %b, i32* %arrayidx1, align 4
+  %arrayidx2 = getelementptr inbounds i32, i32* %d, i32 4
+  store i32 %c, i32* %arrayidx2, align 4
+  tail call void @throws_1(i32 %a, i32 %b, i32 %c) noreturn
+  unreachable
+}
+
+; Don't use STM: there is no available register to store
+; the address. We could transform this with some extra math, but
+; that currently isn't implemented.
+; CHECK-LABEL: no_stm:
+; CHECK: .save   {r7, lr}
+; CHECK: .setfp  r7, sp
+; CHECK: str r0,
+; CHECK: str r1,
+; CHECK: str r2,
+; CHECK: bl throws_2
+define void @no_stm(i32 %a, i32 %b, i32 %c, i32* %d) local_unnamed_addr noreturn "no-frame-pointer-elim"="true" {
+entry:
+  %arrayidx = getelementptr inbounds i32, i32* %d, i32 2
+  store i32 %a, i32* %arrayidx, align 4
+  %arrayidx1 = getelementptr inbounds i32, i32* %d, i32 3
+  store i32 %b, i32* %arrayidx1, align 4
+  %arrayidx2 = getelementptr inbounds i32, i32* %d, i32 4
+  store i32 %c, i32* %arrayidx2, align 4
+  tail call void @throws_2(i32 %a, i32 %b, i32 %c, i32* %d) noreturn
+  unreachable
+}
+
+
+declare void @throws_1(i32, i32, i32) noreturn
+declare void @throws_2(i32, i32, i32, i32*) noreturn
+