[ARM] Allow forcing frame pointers in leaf functions

[olista01]

Summary

This allows the -fno-omit-frame-pointer option (and the
no-frame-pointer-elim function attribute) to work on leaf functions
(those which do not contain any calls).

This still does not emit frame pointers for leaf functions which do not
otherwise need a stack frame, due to the early return in emitPrologue.
This is OK, because a function without a stack frame cannot clobber the
previous value of the frame pointer, so the callee's frame pointer chain
will remain valid.

[rengolin]

I vaguely remember the frame pointer being used by SjLj unwinding, but I don't know enough about SjLj and can't remember what the problem was to say with any accuracy.

[olista01]

Indeed, SjLj EH does use the frame pointer, but doesn't force it to be emitted, and assumes it is always r7! I've raised https://llvm.org/bugs/show_bug.cgi?id=28995 for this.

[olista01]

Now part of D23516 (this patch is required to maintain correctness on iOS/WatchOS with D23516)

lib/Target/ARM/ARMFrameLowering.cpp

bool ARMFrameLowering::hasFP(const MachineFunction &MF) const {
   const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
 
   // ABI-required frame pointer.
   if (hasABIFP(MF))
     return true;
 
   const MachineFrameInfo &MFI = MF.getFrameInfo();
   // Always eliminate non-leaf frame pointers.
-  return ((MF.getTarget().Options.DisableFramePointerElim(MF) &&
-           MFI.hasCalls()) ||
+  return ((MF.getTarget().Options.DisableFramePointerElim(MF)) ||
           RegInfo->needsStackRealignment(MF) ||
           MFI.hasVarSizedObjects() ||
           MFI.isFrameAddressTaken());
 }
 
 /// hasReservedCallFrame - Under normal circumstances, when a frame pointer is
 /// not required, we reserve argument space for call sites in the function
 /// immediately on entry to the current function.  This eliminates the need for

test/CodeGen/Thumb2/frame-pointer.ll

 ; RUN: llc -mtriple=thumbv7m-none-eabi -o - %s | FileCheck %s
 
 declare void @foo()
 
-; Has a call, but no need for a frame pointer.
+; Leaf function, no frame so no need for a frame pointer.
+define void @leaf() {
+; CHECK-LABEL: leaf:
+; CHECK-NOT: push
+; CHECK-NOT: sp
+; CHECK-NOT: pop
+; CHECK: bx lr
+  ret void
+}
+
+; Leaf function, frame pointer is requested but we don't need any stack frame,
+; so don't create a frame pointer.
+define void @leaf_nofpelim() "no-frame-pointer-elim"="true" {
+; CHECK-LABEL: leaf_nofpelim:
+; CHECK-NOT: push
+; CHECK-NOT: sp
+; CHECK-NOT: pop
+; CHECK: bx lr
+  ret void
+}
+
+; Leaf function, frame pointer is requested and we need a stack frame, so we
+; need to use a frame pointer.
+define void @leaf_lowreg_nofpelim() "no-frame-pointer-elim"="true" {
+; CHECK-LABEL: leaf_lowreg_nofpelim:
+; CHECK: push {r4, r7, lr}
+; CHECK: add r7, sp, #4
+; CHECK: pop {r4, r7, pc}
+  call void asm sideeffect "", "~{r4}" ()
+  ret void
+}
+
+; Leaf function, frame pointer is requested and we need a stack frame, so we
+; need to use a frame pointer. A high register is pushed to the stack, so we
+; must use two push/pop instructions to ensure that fp and sp are adjacent on
+; the stack.
+define void @leaf_highreg_nofpelim() "no-frame-pointer-elim"="true" {
+; CHECK-LABEL: leaf_highreg_nofpelim:
+; CHECK: push {r7, lr}
+; CHECK: mov r7, sp
+; CHECK: str r8, [sp, #-4]!
+; CHECK: ldr r8, [sp], #4
+; CHECK: pop {r7, pc}
+  call void asm sideeffect "", "~{r8}" ()
+  ret void
+}
+
+; Leaf function, frame pointer requested for non-leaf functions only, so no
+; need for a stack frame.
+define void @leaf_nononleaffpelim() "no-frame-pointer-elim-non-leaf" {
+; CHECK-LABEL: leaf_nononleaffpelim:
+; CHECK-NOT: push
+; CHECK-NOT: sp
+; CHECK-NOT: pop
+; CHECK: bx lr
+  ret void
+}
+
+; Has a call, but still no need for a frame pointer.
 define void @call() {
 ; CHECK-LABEL: call:
 ; CHECK: push {[[DUMMYREG:r[0-9]+]], lr}
 ; CHECK-NOT: sp
 ; CHECK: bl foo
 ; CHECK: pop {[[DUMMYREG]], pc}
   call void @foo()
   ret void
 }
 
 ; Has a call, and frame pointer requested.
 define void @call_nofpelim() "no-frame-pointer-elim"="true" {
 ; CHECK-LABEL: call_nofpelim:
 ; CHECK: push {r7, lr}
 ; CHECK: mov r7, sp
 ; CHECK: bl foo
 ; CHECK: pop {r7, pc}
   call void @foo()
   ret void
 }
 
+; Has a call, and frame pointer requested for non-leaf function.
+define void @call_nononleaffpelim() "no-frame-pointer-elim-non-leaf" {
+; CHECK-LABEL: call_nononleaffpelim:
+; CHECK: push {r7, lr}
+; CHECK: mov r7, sp
+; CHECK: bl foo
+; CHECK: pop {r7, pc}
+  call void @foo()
+  ret void
+}
+
 ; Has a high register clobbered, no need for a frame pointer.
 define void @highreg() {
 ; CHECK-LABEL: highreg:
 ; CHECK: push.w {r8, lr}
 ; CHECK-NOT: sp
 ; CHECK: bl foo
 ; CHECK: pop.w {r8, pc}
   call void asm sideeffect "", "~{r8}" ()