[WebAssembly] Make tied inline asm operands work again

[aheejin]

Summary

rL341389 broke code with tied register operands in inline assembly. For
example, asm("" : "=r"(var) : "0"(var));
The code above specifies the input operand to be in the same register
with the output operand, tying the two register. This patch makes this
kind of code work again. Without this, this assertion fails.

[sbc100]

I don't know this code myself, but if I'm right in assuming this fixes the current waterfall failures then please commit and we can followup is others have comments.

[aheejin]

• Add comment

[aheejin]

Without this, this assertion fails.

[aheejin]

@dschuff Re: about your question asked in person:
If a reg operand gets tied to another operand in instruction selection, they become the same register in here in TwoAddressInstructionPass. So they are assigned to a same local in this ExplicitLocal pass, so we don't need to worry two tied operands ending up in different locals.

lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp

for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E;) {
         unsigned OldReg = MO.getReg();
 
         // Inline asm may have a def in the middle of the operands. Our contract
         // with inline asm register operands is to provide local indices as
         // immediates.
         if (MO.isDef()) {
           assert(MI.getOpcode() == TargetOpcode::INLINEASM);
           unsigned LocalId = getLocalId(Reg2Local, CurLocal, OldReg);
+          // If this register operand is tied to another operand, we can't
+          // change it to an immediate. Untie it first.
+          MI.untieRegOperand(MI.getOperandNo(&MO));
           MO.ChangeToImmediate(LocalId);
           continue;
         }
 
         // If we see a stackified register, prepare to insert subsequent
         // get_locals before the start of its tree.
         if (MFI.isVRegStackified(OldReg)) {
           InsertPt = findStartOfTree(MO, MRI, MFI);
           continue;
         }
 
         // Our contract with inline asm register operands is to provide local
         // indices as immediates.
         if (MI.getOpcode() == TargetOpcode::INLINEASM) {
           unsigned LocalId = getLocalId(Reg2Local, CurLocal, OldReg);
+          // Untie it first if this reg operand is tied to another operand.
+          MI.untieRegOperand(MI.getOperandNo(&MO));
           MO.ChangeToImmediate(LocalId);
           continue;
         }
 
         // Insert a get_local.
         unsigned LocalId = getLocalId(Reg2Local, CurLocal, OldReg);
         const TargetRegisterClass *RC = MRI.getRegClass(OldReg);
         unsigned NewReg = MRI.createVirtualRegister(RC);

test/CodeGen/WebAssembly/inline-asm.ll

 ; CHECK:      get_local $push{{[0-9]+}}=, [[L2]]{{$}}
 define hidden i32 @r_constraint(i32 %a, i32 %y) {
 entry:
   %z = bitcast i32 0 to i32
   %t0 = tail call i32 asm "foo $0, $1, $2, $3", "=r,r,r,r"(i32 %y, i32 %z, i32 37) #0, !srcloc !0
   ret i32 %t0
 }
 
+; CHECK-LABEL: tied_operands
+; CHECK: get_local  $push0=, 0
+; CHECK: return    $pop0
+define i32 @tied_operands(i32 %var) {
+entry:
+  %ret = call i32 asm "", "=r,0"(i32 %var)
+  ret i32 %ret
+}
+
 attributes #0 = { nounwind }
 
 !0 = !{i32 47}
 !1 = !{i32 145}
 !2 = !{i32 231}