ARM: don't rely on push/pop reglists being in order when folding SP adjust.

It would be a very nice invariant to rely on, but unfortunately it doesn't
necessarily hold (and the causes of mis-sorted reglists appear to be quite
varied) so to be robust the frame lowering code can't assume that the first
register in the list is also the first one that actually gets pushed.

Should fix an issue where we were turning something like:

    push {r8, r4, r7, lr}
    sub sp, #24

into nonsense like:

    push {r2, r3, r4, r5, r6, r7, r8, r4, r7, lr}

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285232 91177308-0d34-0410-b5e6-96231b3b80d8

Author: TNorthover

lib/Target/ARM/ARMBaseInstrInfo.cpp

@@ -2069,29 +2069,40 @@ bool llvm::tryFoldSPUpdateIntoPushPop(const ARMSubtarget &Subtarget,
   int RegListIdx = IsT1PushPop ? 2 : 4;
 
   // Calculate the space we'll need in terms of registers.
-  unsigned FirstReg = MI->getOperand(RegListIdx).getReg();
-  unsigned RD0Reg, RegsNeeded;
+  unsigned RegsNeeded;
+  const TargetRegisterClass *RegClass;
   if (IsVFPPushPop) {
-    RD0Reg = ARM::D0;
     RegsNeeded = NumBytes / 8;
+    RegClass = &ARM::DPRRegClass;
   } else {
-    RD0Reg = ARM::R0;
     RegsNeeded = NumBytes / 4;
+    RegClass = &ARM::GPRRegClass;
   }
 
   // We're going to have to strip all list operands off before
   // re-adding them since the order matters, so save the existing ones
   // for later.
   SmallVector<MachineOperand, 4> RegList;
-  for (int i = MI->getNumOperands() - 1; i >= RegListIdx; --i)
-    RegList.push_back(MI->getOperand(i));
+
+  // We're also going to need the first register transferred by this
+  // instruction, which won't necessarily be the first register in the list.
+  unsigned FirstRegEnc = -1;
 
   const TargetRegisterInfo *TRI = MF.getRegInfo().getTargetRegisterInfo();
+  for (int i = MI->getNumOperands() - 1; i >= RegListIdx; --i) {
+    MachineOperand &MO = MI->getOperand(i);
+    RegList.push_back(MO);
+
+    if (MO.isReg() && TRI->getEncodingValue(MO.getReg()) < FirstRegEnc)
+      FirstRegEnc = TRI->getEncodingValue(MO.getReg());
+  }
+
   const MCPhysReg *CSRegs = TRI->getCalleeSavedRegs(&MF);
 
   // Now try to find enough space in the reglist to allocate NumBytes.
-  for (unsigned CurReg = FirstReg - 1; CurReg >= RD0Reg && RegsNeeded;
-       --CurReg) {
+  for (int CurRegEnc = FirstRegEnc - 1; CurRegEnc >= 0 && RegsNeeded;
+       --CurRegEnc) {
+    unsigned CurReg = RegClass->getRegister(CurRegEnc);
     if (!IsPop) {
       // Pushing any register is completely harmless, mark the
       // register involved as undef since we don't care about it in

test/CodeGen/ARM/fold-stack-adjust.ll

@@ -218,4 +218,18 @@ exit:                                             ; preds = %if.then, %entry
   ret float %call1
 }
 
+declare void @use_arr(i32*)
+define void @test_fold_reuse() minsize {
+; CHECK-LABEL: test_fold_reuse:
+; CHECK: push {r4, r7, lr}
+; CHECK: sub sp, #24
+; [...]
+; CHECK: add sp, #24
+; CHECK: pop {r4, r7, pc}
+  %arr = alloca i8, i32 24
+  call void asm sideeffect "", "~{r8},~{r4}"()
+  call void @bar(i8* %arr)
+  ret void
+}
+
 declare void @llvm.va_start(i8*) nounwind