[WebAssembly][InstrEmitter] Foundation for multivalue call lowering

Summary:
WebAssembly is unique among upstream targets in that it does not at
any point use physical registers to store values. Instead, it uses
virtual registers to model positions in its value stack. This means
that some target-independent lowering activities that would use
physical registers need to use virtual registers instead for
WebAssembly and similar downstream targets. This CL generalizes the
existing `usesPhysRegsForPEI` lowering hook to
`usesPhysRegsForValues` in preparation for using it in more places.

One such place is in InstrEmitter for instructions that have variadic
defs. On register machines, it only makes sense for these defs to be
physical registers, but for WebAssembly they must be virtual registers
like any other values. This CL changes InstrEmitter to check the new
target lowering hook to determine whether variadic defs should be
physical or virtual registers.

These changes are necessary to support a generalized CALL instruction
for WebAssembly that is capable of returning an arbitrary number of
arguments. Fully implementing that instruction will require additional
changes that are described in comments here but left for a follow up
commit.

Reviewers: aheejin, dschuff, qcolombet

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71484

1 files changed, 4 insertions, 5 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
index 421d353..58082bb 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
@@ -135,12 +135,12 @@ static void convertImplicitDefToConstZero(MachineInstr *MI,
 // Determine whether a call to the callee referenced by
 // MI->getOperand(CalleeOpNo) reads memory, writes memory, and/or has side
 // effects.
-static void queryCallee(const MachineInstr &MI, unsigned CalleeOpNo, bool &Read,
-                        bool &Write, bool &Effects, bool &StackPointer) {
+static void queryCallee(const MachineInstr &MI, bool &Read, bool &Write,
+                        bool &Effects, bool &StackPointer) {
   // All calls can use the stack pointer.
   StackPointer = true;
 
-  const MachineOperand &MO = MI.getOperand(CalleeOpNo);
+  const MachineOperand &MO = WebAssembly::getCalleeOp(MI);
   if (MO.isGlobal()) {
     const Constant *GV = MO.getGlobal();
     if (const auto *GA = dyn_cast<GlobalAlias>(GV))
@@ -252,8 +252,7 @@ static void query(const MachineInstr &MI, AliasAnalysis &AA, bool &Read,
 
   // Analyze calls.
   if (MI.isCall()) {
-    unsigned CalleeOpNo = WebAssembly::getCalleeOpNo(MI.getOpcode());
-    queryCallee(MI, CalleeOpNo, Read, Write, Effects, StackPointer);
+    queryCallee(MI, Read, Write, Effects, StackPointer);
   }
 }