compiler,runtime: translate memzero calls to LLVM memset intrinsics

This gives the optimizer a bit more information about what the calls do.
This should result in slightly better generated code.

Code size sometimes goes up and sometimes goes down. I blame the code
size going up on the inliner which inlines more functions, because
compiling the smoke tests in the drivers repository with -opt=1 results
in a slight code size reduction in all cases.

compiler/compiler.go

@@ -1343,6 +1343,8 @@ func (b *builder) createFunctionCall(instr *ssa.CallCommon) (llvm.Value, error)
 		switch {
 		case name == "runtime.memcpy" || name == "runtime.memmove" || name == "reflect.memcpy":
 			return b.createMemoryCopyCall(fn, instr.Args)
+		case name == "runtime.memzero":
+			return b.createMemoryZeroCall(instr.Args)
 		case name == "device/arm.ReadRegister" || name == "device/riscv.ReadRegister":
 			return b.createReadRegister(name, instr.Args)
 		case name == "device/arm.Asm" || name == "device/avr.Asm" || name == "device/riscv.Asm":

compiler/intrinsics.go

@@ -28,3 +28,23 @@ func (b *builder) createMemoryCopyCall(fn *ssa.Function, args []ssa.Value) (llvm
 	b.CreateCall(llvmFn, params, "")
 	return llvm.Value{}, nil
 }
+
+// createMemoryZeroCall creates calls to llvm.memset.* to zero a block of
+// memory, declaring the function if needed. These calls will be lowered to
+// regular libc memset calls if they aren't optimized out in a different way.
+func (b *builder) createMemoryZeroCall(args []ssa.Value) (llvm.Value, error) {
+	fnName := "llvm.memset.p0i8.i" + strconv.Itoa(b.uintptrType.IntTypeWidth())
+	llvmFn := b.mod.NamedFunction(fnName)
+	if llvmFn.IsNil() {
+		fnType := llvm.FunctionType(b.ctx.VoidType(), []llvm.Type{b.i8ptrType, b.ctx.Int8Type(), b.uintptrType, b.ctx.Int1Type()}, false)
+		llvmFn = llvm.AddFunction(b.mod, fnName, fnType)
+	}
+	params := []llvm.Value{
+		b.getValue(args[0]),
+		llvm.ConstInt(b.ctx.Int8Type(), 0, false),
+		b.getValue(args[1]),
+		llvm.ConstInt(b.ctx.Int1Type(), 0, false),
+	}
+	b.CreateCall(llvmFn, params, "")
+	return llvm.Value{}, nil
+}

src/runtime/runtime.go

@@ -41,11 +41,9 @@ func memcpy(dst, src unsafe.Pointer, size uintptr)
 func memmove(dst, src unsafe.Pointer, size uintptr)
 
 // Set the given number of bytes to zero.
-func memzero(ptr unsafe.Pointer, size uintptr) {
-	for i := uintptr(0); i < size; i++ {
-		*(*byte)(unsafe.Pointer(uintptr(ptr) + i)) = 0
-	}
-}
+// Calls to this function are converted to LLVM intrinsic calls such as
+// llvm.memset.p0i8.i32(ptr, 0, size, false).
+func memzero(ptr unsafe.Pointer, size uintptr)
 
 // Compare two same-size buffers for equality.
 func memequal(x, y unsafe.Pointer, n uintptr) bool {