ARM/Dwarf: correctly align stack before callee-saved VPRs

We were making an attempt to do this by adding an extra callee-saved GPR (so
that there was an even number in the list), but when that failed we went ahead
and pushed anyway.

This had a couple of potential issues:
  + The .cfi directives we emit misplaced dN because they were based on
    PrologEpilogInserter's calculation.
  + Unaligned stores can be less efficient.
  + Unaligned stores can actually fault (likely only an issue in niche cases,
    but possible).

This adds a final explicit stack adjustment if all other options fail, so that
the actual locations of the registers match up with where they should be.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221320 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/ARM/ARMFrameLowering.cpp

@@ -260,10 +260,11 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
 
   // Determine starting offsets of spill areas.
   bool HasFP = hasFP(MF);
-  unsigned DPRCSOffset  = NumBytes - (ArgRegsSaveSize + GPRCS1Size
-                                      + GPRCS2Size + DPRCSSize);
-  unsigned GPRCS2Offset = DPRCSOffset + DPRCSSize;
-  unsigned GPRCS1Offset = GPRCS2Offset + GPRCS2Size;
+  unsigned GPRCS1Offset = NumBytes - ArgRegsSaveSize - GPRCS1Size;
+  unsigned GPRCS2Offset = GPRCS1Offset - GPRCS2Size;
+  unsigned DPRAlign = DPRCSSize ? std::min(8U, Align) : 4U;
+  unsigned DPRGapSize = (GPRCS1Size + GPRCS2Size + ArgRegsSaveSize) % DPRAlign;
+  unsigned DPRCSOffset = GPRCS2Offset - DPRGapSize - DPRCSSize;
   int FramePtrOffsetInPush = 0;
   if (HasFP) {
     FramePtrOffsetInPush = MFI->getObjectOffset(FramePtrSpillFI)
@@ -279,6 +280,15 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
   if (GPRCS2Size > 0)
     GPRCS2Push = LastPush = MBBI++;
 
+  // Prolog/epilog inserter assumes we correctly align DPRs on the stack, so our
+  // .cfi_offset operations will reflect that.
+  if (DPRGapSize) {
+    assert(DPRGapSize == 4 && "unexpected alignment requirements for DPRs");
+    if (!tryFoldSPUpdateIntoPushPop(STI, MF, LastPush, DPRGapSize))
+      emitSPUpdate(isARM, MBB, MBBI, dl, TII, -DPRGapSize,
+                   MachineInstr::FrameSetup);
+  }
+
   // Move past area 3.
   if (DPRCSSize > 0) {
     DPRCSPush = MBBI;
@@ -508,6 +518,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
 
   AFI->setGPRCalleeSavedArea1Size(GPRCS1Size);
   AFI->setGPRCalleeSavedArea2Size(GPRCS2Size);
+  AFI->setDPRCalleeSavedGapSize(DPRGapSize);
   AFI->setDPRCalleeSavedAreaSize(DPRCSSize);
 
   // If we need dynamic stack realignment, do it here. Be paranoid and make
@@ -613,6 +624,7 @@ void ARMFrameLowering::emitEpilogue(MachineFunction &MF,
     NumBytes -= (ArgRegsSaveSize +
                  AFI->getGPRCalleeSavedArea1Size() +
                  AFI->getGPRCalleeSavedArea2Size() +
+                 AFI->getDPRCalleeSavedGapSize() +
                  AFI->getDPRCalleeSavedAreaSize());
 
     // Reset SP based on frame pointer only if the stack frame extends beyond
@@ -661,6 +673,12 @@ void ARMFrameLowering::emitEpilogue(MachineFunction &MF,
       while (MBBI->getOpcode() == ARM::VLDMDIA_UPD)
         MBBI++;
     }
+    if (AFI->getDPRCalleeSavedGapSize()) {
+      assert(AFI->getDPRCalleeSavedGapSize() == 4 &&
+             "unexpected DPR alignment gap");
+      emitSPUpdate(isARM, MBB, MBBI, dl, TII, AFI->getDPRCalleeSavedGapSize());
+    }
+
     if (AFI->getGPRCalleeSavedArea2Size()) MBBI++;
     if (AFI->getGPRCalleeSavedArea1Size()) MBBI++;
   }

lib/Target/ARM/ARMMachineFunctionInfo.h

@@ -86,6 +86,7 @@ class ARMFunctionInfo : public MachineFunctionInfo {
   /// areas.
   unsigned GPRCS1Size;
   unsigned GPRCS2Size;
+  unsigned DPRCSAlignGapSize;
   unsigned DPRCSSize;
 
   /// NumAlignedDPRCS2Regs - The number of callee-saved DPRs that are saved in
@@ -134,7 +135,7 @@ public:
     RestoreSPFromFP(false),
     LRSpilledForFarJump(false),
     FramePtrSpillOffset(0), GPRCS1Offset(0), GPRCS2Offset(0), DPRCSOffset(0),
-    GPRCS1Size(0), GPRCS2Size(0), DPRCSSize(0),
+    GPRCS1Size(0), GPRCS2Size(0), DPRCSAlignGapSize(0), DPRCSSize(0),
     NumAlignedDPRCS2Regs(0),
     JumpTableUId(0), PICLabelUId(0),
     VarArgsFrameIndex(0), HasITBlocks(false), GlobalBaseReg(0) {}
@@ -183,10 +184,12 @@ public:
 
   unsigned getGPRCalleeSavedArea1Size() const { return GPRCS1Size; }
   unsigned getGPRCalleeSavedArea2Size() const { return GPRCS2Size; }
+  unsigned getDPRCalleeSavedGapSize() const   { return DPRCSAlignGapSize; }
   unsigned getDPRCalleeSavedAreaSize()  const { return DPRCSSize; }
 
   void setGPRCalleeSavedArea1Size(unsigned s) { GPRCS1Size = s; }
   void setGPRCalleeSavedArea2Size(unsigned s) { GPRCS2Size = s; }
+  void setDPRCalleeSavedGapSize(unsigned s)   { DPRCSAlignGapSize = s; }
   void setDPRCalleeSavedAreaSize(unsigned s)  { DPRCSSize = s; }
 
   unsigned getArgumentStackSize() const { return ArgumentStackSize; }

test/CodeGen/ARM/dwarf-unwind.ll

@@ -0,0 +1,68 @@
+; RUN: llc -mtriple=thumbv7-netbsd-eabi -o - %s | FileCheck %s
+declare void @bar()
+
+; ARM's frame lowering attempts to tack another callee-saved register onto the
+; list when it detects a potential misaligned VFP store. However, if there are
+; none available it used to just vpush anyway and misreport the location of the
+; registers in unwind info. Since there are benefits to aligned stores, it's
+; better to correct the code than the .cfi_offset directive.
+
+define void @test_dpr_align(i8 %l, i8 %r) {
+; CHECK-LABEL: test_dpr_align:
+; CHECK: push.w {r4, r5, r6, r7, r8, r9, r10, r11, lr}
+; CHECK: sub sp, #4
+; CHECK: vpush {d8}
+; CHECK: .cfi_offset d8, -48
+; CHECK-NOT: sub sp
+; [...]
+; CHECK: bl bar
+; CHECK-NOT: add sp
+; CHECK: vpop {d8}
+; CHECK: add sp, #4
+; CHECK: pop.w {r4, r5, r6, r7, r8, r9, r10, r11, pc}
+  call void asm sideeffect "", "~{r4},~{r5},~{r6},~{r7},~{r8},~{r9},~{r10},~{r11},~{d8}"()
+  call void @bar()
+  ret void
+}
+
+; The prologue (but not the epilogue) can be made more space efficient by
+; chucking an argument register into the list. Not worth it in general though,
+; "sub sp, #4" is likely faster.
+define void @test_dpr_align_tiny(i8 %l, i8 %r) minsize {
+; CHECK-LABEL: test_dpr_align_tiny:
+; CHECK: push.w {r3, r4, r5, r6, r7, r8, r9, r10, r11, lr}
+; CHECK-NOT: sub sp
+; CHECK: vpush {d8}
+; CHECK: .cfi_offset d8, -48
+; CHECK-NOT: sub sp
+; [...]
+; CHECK: bl bar
+; CHECK-NOT: add sp
+; CHECK: vpop {d8}
+; CHECK: add sp, #4
+; CHECK: pop.w {r4, r5, r6, r7, r8, r9, r10, r11, pc}
+  call void asm sideeffect "", "~{r4},~{r5},~{r6},~{r7},~{r8},~{r9},~{r10},~{r11},~{d8}"()
+  call void @bar()
+  ret void
+}
+
+
+; However, we shouldn't do a 2-step align/adjust if there are no DPRs to be
+; saved.
+define void @test_nodpr_noalign(i8 %l, i8 %r) {
+; CHECK-LABEL: test_nodpr_noalign:
+; CHECK: push.w {r4, r5, r6, r7, r8, r9, r10, r11, lr}
+; CHECK-NOT: sub sp
+; CHECK: sub sp, #12
+; CHECK-NOT: sub sp
+; [...]
+; CHECK: bl bar
+; CHECK-NOT: add sp
+; CHECK: add sp, #12
+; CHECK-NOT: add sp
+; CHECK: pop.w {r4, r5, r6, r7, r8, r9, r10, r11, pc}
+  alloca i64
+  call void asm sideeffect "", "~{r4},~{r5},~{r6},~{r7},~{r8},~{r9},~{r10},~{r11}"()
+  call void @bar()
+  ret void
+}