[WebAssembly] Emit a BasePointer when we have overly-aligned stack objects
Because we shift the stack pointer by an unknown amount, we need an
additional pointer. In the case where we have variable-size objects
as well, we can't reuse the frame pointer, thus three pointers.
Patch by Jacob Gravelle
Differential Revision: https://reviews.llvm.org/D26263
llvm-svn: 286160
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp
index a1c9658..25b203a 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp
@@ -37,15 +37,34 @@
// TODO: wasm64
// TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
+/// We need a base pointer in the case of having items on the stack that
+/// require stricter alignment than the stack pointer itself. Because we need
+/// to shift the stack pointer by some unknown amount to force the alignment,
+/// we need to record the value of the stack pointer on entry to the function.
+bool WebAssemblyFrameLowering::hasBP(
+ const MachineFunction &MF) const {
+ const auto *RegInfo =
+ MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
+ return RegInfo->needsStackRealignment(MF);
+}
+
/// Return true if the specified function should have a dedicated frame pointer
/// register.
bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const {
const MachineFrameInfo &MFI = MF.getFrameInfo();
- const auto *RegInfo =
- MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
- return MFI.isFrameAddressTaken() || MFI.hasVarSizedObjects() ||
- MFI.hasStackMap() || MFI.hasPatchPoint() ||
- RegInfo->needsStackRealignment(MF);
+
+ // When we have var-sized objects, we move the stack pointer by an unknown
+ // amount, and need to emit a frame pointer to restore the stack to where we
+ // were on function entry.
+ // If we already need a base pointer, we use that to fix up the stack pointer.
+ // If there are no fixed-size objects, we would have no use of a frame
+ // pointer, and thus should not emit one.
+ bool HasFixedSizedObjects = MFI.getStackSize() > 0;
+ bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects;
+
+ return MFI.isFrameAddressTaken() ||
+ (MFI.hasVarSizedObjects() && NeedsFixedReference) ||
+ MFI.hasStackMap() || MFI.hasPatchPoint();
}
/// Under normal circumstances, when a frame pointer is not required, we reserve
@@ -107,7 +126,7 @@
WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
MachineFunction &MF, MachineBasicBlock &MBB,
MachineBasicBlock::iterator I) const {
- assert(!I->getOperand(0).getImm() && hasFP(MF) &&
+ assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) &&
"Call frame pseudos should only be used for dynamic stack adjustment");
const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
@@ -137,7 +156,9 @@
const TargetRegisterClass *PtrRC =
MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
unsigned Zero = MRI.createVirtualRegister(PtrRC);
- unsigned SPReg = MRI.createVirtualRegister(PtrRC);
+ unsigned SPReg = WebAssembly::SP32;
+ if (StackSize)
+ SPReg = MRI.createVirtualRegister(PtrRC);
const char *ES = "__stack_pointer";
auto *SPSymbol = MF.createExternalSymbolName(ES);
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), Zero)
@@ -146,13 +167,20 @@
MachinePointerInfo(MF.getPSVManager().getExternalSymbolCallEntry(ES)),
MachineMemOperand::MOLoad, 4, 4);
// Load the SP value.
- BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32),
- StackSize ? SPReg : (unsigned)WebAssembly::SP32)
+ BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32), SPReg)
.addImm(2) // p2align
.addExternalSymbol(SPSymbol)
.addReg(Zero) // addr
.addMemOperand(LoadMMO);
+ bool HasBP = hasBP(MF);
+ if (HasBP) {
+ auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
+ unsigned BasePtr = MRI.createVirtualRegister(PtrRC);
+ FI->setBasePointerVreg(BasePtr);
+ BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
+ .addReg(SPReg);
+ }
if (StackSize) {
// Subtract the frame size
unsigned OffsetReg = MRI.createVirtualRegister(PtrRC);
@@ -163,6 +191,18 @@
.addReg(SPReg)
.addReg(OffsetReg);
}
+ if (HasBP) {
+ unsigned BitmaskReg = MRI.createVirtualRegister(PtrRC);
+ unsigned Alignment = MFI.getMaxAlignment();
+ assert((1 << countTrailingZeros(Alignment)) == Alignment &&
+ "Alignment must be a power of 2");
+ BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), BitmaskReg)
+ .addImm((int)~(Alignment - 1));
+ BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::AND_I32),
+ WebAssembly::SP32)
+ .addReg(WebAssembly::SP32)
+ .addReg(BitmaskReg);
+ }
if (hasFP(MF)) {
// Unlike most conventional targets (where FP points to the saved FP),
// FP points to the bottom of the fixed-size locals, so we can use positive
@@ -193,7 +233,10 @@
// subtracted in the prolog.
unsigned SPReg = 0;
MachineBasicBlock::iterator InsertAddr = InsertPt;
- if (StackSize) {
+ if (hasBP(MF)) {
+ auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
+ SPReg = FI->getBasePointerVreg();
+ } else if (StackSize) {
const TargetRegisterClass *PtrRC =
MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
unsigned OffsetReg = MRI.createVirtualRegister(PtrRC);