[WebAssembly][FastISel] Emit signed loads for sext of i8/i16/i32 (llvm#182767)

FastISel currently defaults to unsigned loads for i8/i16/i32 types,
leaving any sign-extension to be handled by a separate instruction. This
patch optimizes this by folding the SExtInst into the LoadInst, directly
emitting a signed load (e.g., i32.load8_s).

When a load has a single SExtInst use, selectLoad emits a signed load
and safely removes the redundantly emitted SExtInst.

Fixed: llvm#180783

Author: ParkHanbum

llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp

@@ -204,6 +204,8 @@ class WebAssemblyFastISel final : public FastISel {
   }
 
   bool fastSelectInstruction(const Instruction *I) override;
+  bool tryToFoldLoadIntoMI(MachineInstr *MI, unsigned OpNo,
+                           const LoadInst *LI) override;
 
 #include "WebAssemblyGenFastISel.inc"
 };
@@ -1267,6 +1269,52 @@ bool WebAssemblyFastISel::selectBitCast(const Instruction *I) {
   return true;
 }
 
+static unsigned getSExtLoadOpcode(unsigned Opc, bool A64) {
+  switch (Opc) {
+  default:
+    return WebAssembly::INSTRUCTION_LIST_END;
+  case WebAssembly::I32_EXTEND8_S_I32:
+    Opc = A64 ? WebAssembly::LOAD8_S_I32_A64 : WebAssembly::LOAD8_S_I32_A32;
+    break;
+  case WebAssembly::I32_EXTEND16_S_I32:
+    Opc = A64 ? WebAssembly::LOAD16_S_I32_A64 : WebAssembly::LOAD16_S_I32_A32;
+    break;
+  case WebAssembly::I64_EXTEND8_S_I64:
+    Opc = A64 ? WebAssembly::LOAD8_S_I64_A64 : WebAssembly::LOAD8_S_I64_A32;
+    break;
+  case WebAssembly::I64_EXTEND16_S_I64:
+    Opc = A64 ? WebAssembly::LOAD16_S_I64_A64 : WebAssembly::LOAD16_S_I64_A32;
+    break;
+  case WebAssembly::I64_EXTEND32_S_I64:
+  case WebAssembly::I64_EXTEND_S_I32:
+    Opc = A64 ? WebAssembly::LOAD32_S_I64_A64 : WebAssembly::LOAD32_S_I64_A32;
+    break;
+  }
+
+  return Opc;
+}
+
+bool WebAssemblyFastISel::tryToFoldLoadIntoMI(MachineInstr *MI, unsigned OpNo,
+                                              const LoadInst *LI) {
+  bool A64 = Subtarget->hasAddr64();
+  unsigned NewOpc;
+  if ((NewOpc = getSExtLoadOpcode(MI->getOpcode(), A64)) ==
+      WebAssembly::INSTRUCTION_LIST_END)
+    return false;
+
+  Address Addr;
+  if (!computeAddress(LI->getPointerOperand(), Addr))
+    return false;
+
+  Register ResultReg = MI->getOperand(0).getReg();
+  MachineInstrBuilder MIB = BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, MIMD,
+                                    TII.get(NewOpc), ResultReg);
+  addLoadStoreOperands(Addr, MIB, createMachineMemOperandFor(LI));
+  MachineBasicBlock::iterator Iter(MI);
+  removeDeadCode(Iter, std::next(Iter));
+  return true;
+}
+
 bool WebAssemblyFastISel::selectLoad(const Instruction *I) {
   const auto *Load = cast<LoadInst>(I);
   if (Load->isAtomic())

llvm/test/CodeGen/WebAssembly/load-ext.ll

@@ -26,8 +26,7 @@ define i32 @sext_i8_i32(ptr %p) {
 ; WASM32-FAST-LABEL: sext_i8_i32:
 ; WASM32-FAST:         .functype sext_i8_i32 (i32) -> (i32)
 ; WASM32-FAST-NEXT:  # %bb.0:
-; WASM32-FAST-NEXT:    i32.load8_u $push1=, 0($0)
-; WASM32-FAST-NEXT:    i32.extend8_s $push0=, $pop1
+; WASM32-FAST-NEXT:    i32.load8_s $push0=, 0($0)
 ; WASM32-FAST-NEXT:    return $pop0
 ;
 ; WASM32-FAST-MVP-LABEL: sext_i8_i32:
@@ -55,8 +54,7 @@ define i32 @sext_i8_i32(ptr %p) {
 ; WASM64-FAST-LABEL: sext_i8_i32:
 ; WASM64-FAST:         .functype sext_i8_i32 (i64) -> (i32)
 ; WASM64-FAST-NEXT:  # %bb.0:
-; WASM64-FAST-NEXT:    i32.load8_u $push1=, 0($0)
-; WASM64-FAST-NEXT:    i32.extend8_s $push0=, $pop1
+; WASM64-FAST-NEXT:    i32.load8_s $push0=, 0($0)
 ; WASM64-FAST-NEXT:    return $pop0
 ;
 ; WASM64-FAST-MVP-LABEL: sext_i8_i32:
@@ -150,8 +148,7 @@ define i32 @sext_i16_i32(ptr %p) {
 ; WASM32-FAST-LABEL: sext_i16_i32:
 ; WASM32-FAST:         .functype sext_i16_i32 (i32) -> (i32)
 ; WASM32-FAST-NEXT:  # %bb.0:
-; WASM32-FAST-NEXT:    i32.load16_u $push1=, 0($0)
-; WASM32-FAST-NEXT:    i32.extend16_s $push0=, $pop1
+; WASM32-FAST-NEXT:    i32.load16_s $push0=, 0($0)
 ; WASM32-FAST-NEXT:    return $pop0
 ;
 ; WASM32-FAST-MVP-LABEL: sext_i16_i32:
@@ -179,8 +176,7 @@ define i32 @sext_i16_i32(ptr %p) {
 ; WASM64-FAST-LABEL: sext_i16_i32:
 ; WASM64-FAST:         .functype sext_i16_i32 (i64) -> (i32)
 ; WASM64-FAST-NEXT:  # %bb.0:
-; WASM64-FAST-NEXT:    i32.load16_u $push1=, 0($0)
-; WASM64-FAST-NEXT:    i32.extend16_s $push0=, $pop1
+; WASM64-FAST-NEXT:    i32.load16_s $push0=, 0($0)
 ; WASM64-FAST-NEXT:    return $pop0
 ;
 ; WASM64-FAST-MVP-LABEL: sext_i16_i32:
@@ -538,8 +534,7 @@ define i64 @sext_i32_i64(ptr %p) {
 ; WASM32-FAST-LABEL: sext_i32_i64:
 ; WASM32-FAST:         .functype sext_i32_i64 (i32) -> (i64)
 ; WASM32-FAST-NEXT:  # %bb.0:
-; WASM32-FAST-NEXT:    i32.load $push1=, 0($0)
-; WASM32-FAST-NEXT:    i64.extend_i32_s $push0=, $pop1
+; WASM32-FAST-NEXT:    i64.load32_s $push0=, 0($0)
 ; WASM32-FAST-NEXT:    return $pop0
 ;
 ; WASM32-FAST-MVP-LABEL: sext_i32_i64:
@@ -564,8 +559,7 @@ define i64 @sext_i32_i64(ptr %p) {
 ; WASM64-FAST-LABEL: sext_i32_i64:
 ; WASM64-FAST:         .functype sext_i32_i64 (i64) -> (i64)
 ; WASM64-FAST-NEXT:  # %bb.0:
-; WASM64-FAST-NEXT:    i32.load $push1=, 0($0)
-; WASM64-FAST-NEXT:    i64.extend_i32_s $push0=, $pop1
+; WASM64-FAST-NEXT:    i64.load32_s $push0=, 0($0)
 ; WASM64-FAST-NEXT:    return $pop0
 ;
 ; WASM64-FAST-MVP-LABEL: sext_i32_i64:

llvm/test/CodeGen/WebAssembly/offset-fastisel.ll

@@ -121,8 +121,7 @@ define i32 @load_i8_s_with_folded_offset(ptr %p) {
 ; DEFAULT-LABEL: load_i8_s_with_folded_offset:
 ; DEFAULT:         .functype load_i8_s_with_folded_offset (i32) -> (i32)
 ; DEFAULT-NEXT:  # %bb.0:
-; DEFAULT-NEXT:    i32.load8_u $push1=, 24($0)
-; DEFAULT-NEXT:    i32.extend8_s $push0=, $pop1
+; DEFAULT-NEXT:    i32.load8_s $push0=, 24($0)
 ; DEFAULT-NEXT:    # fallthrough-return
 ;
 ; MVP-LABEL: load_i8_s_with_folded_offset: