[WebAssembly] Optimize lowering of constant-sized memcpy and memset (#163294)

We currently emit a check that the size operand isn't zero, to avoid
executing the wasm memory.copy instruction when it would trap.
But this isn't necessary if the operand is a constant.

Fixes #163245

1 files changed, 45 insertions, 0 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
index 47c24fc..f973949 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -601,6 +601,29 @@ static MachineBasicBlock *LowerMemcpy(MachineInstr &MI, DebugLoc DL,
   MachineOperand Src = MI.getOperand(3);
   MachineOperand Len = MI.getOperand(4);
 
+  // If the length is a constant, we don't actually need the check.
+  if (MachineInstr *Def = MRI.getVRegDef(Len.getReg())) {
+    if (Def->getOpcode() == WebAssembly::CONST_I32 ||
+        Def->getOpcode() == WebAssembly::CONST_I64) {
+      if (Def->getOperand(1).getImm() == 0) {
+        // A zero-length memcpy is a no-op.
+        MI.eraseFromParent();
+        return BB;
+      }
+      // A non-zero-length memcpy doesn't need a zero check.
+      unsigned MemoryCopy =
+          Int64 ? WebAssembly::MEMORY_COPY_A64 : WebAssembly::MEMORY_COPY_A32;
+      BuildMI(*BB, MI, DL, TII.get(MemoryCopy))
+          .add(DstMem)
+          .add(SrcMem)
+          .add(Dst)
+          .add(Src)
+          .add(Len);
+      MI.eraseFromParent();
+      return BB;
+    }
+  }
+
   // We're going to add an extra use to `Len` to test if it's zero; that
   // use shouldn't be a kill, even if the original use is.
   MachineOperand NoKillLen = Len;
@@ -669,6 +692,28 @@ static MachineBasicBlock *LowerMemset(MachineInstr &MI, DebugLoc DL,
   MachineOperand Val = MI.getOperand(2);
   MachineOperand Len = MI.getOperand(3);
 
+  // If the length is a constant, we don't actually need the check.
+  if (MachineInstr *Def = MRI.getVRegDef(Len.getReg())) {
+    if (Def->getOpcode() == WebAssembly::CONST_I32 ||
+        Def->getOpcode() == WebAssembly::CONST_I64) {
+      if (Def->getOperand(1).getImm() == 0) {
+        // A zero-length memset is a no-op.
+        MI.eraseFromParent();
+        return BB;
+      }
+      // A non-zero-length memset doesn't need a zero check.
+      unsigned MemoryFill =
+          Int64 ? WebAssembly::MEMORY_FILL_A64 : WebAssembly::MEMORY_FILL_A32;
+      BuildMI(*BB, MI, DL, TII.get(MemoryFill))
+          .add(Mem)
+          .add(Dst)
+          .add(Val)
+          .add(Len);
+      MI.eraseFromParent();
+      return BB;
+    }
+  }
+
   // We're going to add an extra use to `Len` to test if it's zero; that
   // use shouldn't be a kill, even if the original use is.
   MachineOperand NoKillLen = Len;