compiler: implement most math/bits functions

These functions can be implemented more efficiently using LLVM
intrinsics. That makes them the Go equivalent of functions like
__builtin_clz which are also implemented using these LLVM intrinsics.

I believe the Go compiler does something very similar: IIRC it converts
calls to these functions into optimal instructions for the given
architecture.

I tested these by running `tinygo test math/bits` after uncommenting the
tests that would always fail (the *PanicZero and *PanicOverflow tests).

compiler/compiler.go

@@ -845,6 +845,11 @@ func (c *compilerContext) createPackage(irbuilder llvm.Builder, pkg *ssa.Package
 				b.defineMathOp()
 				continue
 			}
+			if ok := b.defineMathBitsIntrinsic(); ok {
+				// Like a math intrinsic, the body of this function was replaced
+				// with a LLVM intrinsic.
+				continue
+			}
 			if member.Blocks == nil {
 				// Try to define this as an intrinsic function.
 				b.defineIntrinsicFunction()

compiler/intrinsics.go

@@ -154,3 +154,118 @@ func (b *builder) defineMathOp() {
 	result := b.CreateCall(llvmFn.GlobalValueType(), llvmFn, args, "")
 	b.CreateRet(result)
 }
+
+// Implement most math/bits functions.
+//
+// This implements all the functions that operate on bits. It does not yet
+// implement the arithmetic functions (like bits.Add), which also have LLVM
+// intrinsics.
+func (b *builder) defineMathBitsIntrinsic() bool {
+	if b.fn.Pkg.Pkg.Path() != "math/bits" {
+		return false
+	}
+	name := b.fn.Name()
+	switch name {
+	case "LeadingZeros", "LeadingZeros8", "LeadingZeros16", "LeadingZeros32", "LeadingZeros64",
+		"TrailingZeros", "TrailingZeros8", "TrailingZeros16", "TrailingZeros32", "TrailingZeros64":
+		b.createFunctionStart(true)
+		param := b.getValue(b.fn.Params[0], b.fn.Pos())
+		valueType := param.Type()
+		var intrinsicName string
+		if strings.HasPrefix(name, "Leading") { // LeadingZeros
+			intrinsicName = "llvm.ctlz.i" + strconv.Itoa(valueType.IntTypeWidth())
+		} else { // TrailingZeros
+			intrinsicName = "llvm.cttz.i" + strconv.Itoa(valueType.IntTypeWidth())
+		}
+		llvmFn := b.mod.NamedFunction(intrinsicName)
+		llvmFnType := llvm.FunctionType(valueType, []llvm.Type{valueType, b.ctx.Int1Type()}, false)
+		if llvmFn.IsNil() {
+			llvmFn = llvm.AddFunction(b.mod, intrinsicName, llvmFnType)
+		}
+		result := b.createCall(llvmFnType, llvmFn, []llvm.Value{
+			param,
+			llvm.ConstInt(b.ctx.Int1Type(), 0, false),
+		}, "")
+		result = b.createZExtOrTrunc(result, b.intType)
+		b.CreateRet(result)
+		return true
+	case "Len", "Len8", "Len16", "Len32", "Len64":
+		// bits.Len can be implemented as:
+		//     (unsafe.Sizeof(v) * 8) -  bits.LeadingZeros(n)
+		// Not sure why this isn't already done in the standard library, as it
+		// is much simpler than a lookup table.
+		b.createFunctionStart(true)
+		param := b.getValue(b.fn.Params[0], b.fn.Pos())
+		valueType := param.Type()
+		valueBits := valueType.IntTypeWidth()
+		intrinsicName := "llvm.ctlz.i" + strconv.Itoa(valueBits)
+		llvmFn := b.mod.NamedFunction(intrinsicName)
+		llvmFnType := llvm.FunctionType(valueType, []llvm.Type{valueType, b.ctx.Int1Type()}, false)
+		if llvmFn.IsNil() {
+			llvmFn = llvm.AddFunction(b.mod, intrinsicName, llvmFnType)
+		}
+		result := b.createCall(llvmFnType, llvmFn, []llvm.Value{
+			param,
+			llvm.ConstInt(b.ctx.Int1Type(), 0, false),
+		}, "")
+		result = b.createZExtOrTrunc(result, b.intType)
+		maxLen := llvm.ConstInt(b.intType, uint64(valueBits), false) // number of bits in the value
+		result = b.CreateSub(maxLen, result, "")
+		b.CreateRet(result)
+		return true
+	case "OnesCount", "OnesCount8", "OnesCount16", "OnesCount32", "OnesCount64":
+		b.createFunctionStart(true)
+		param := b.getValue(b.fn.Params[0], b.fn.Pos())
+		valueType := param.Type()
+		intrinsicName := "llvm.ctpop.i" + strconv.Itoa(valueType.IntTypeWidth())
+		llvmFn := b.mod.NamedFunction(intrinsicName)
+		llvmFnType := llvm.FunctionType(valueType, []llvm.Type{valueType}, false)
+		if llvmFn.IsNil() {
+			llvmFn = llvm.AddFunction(b.mod, intrinsicName, llvmFnType)
+		}
+		result := b.createCall(llvmFnType, llvmFn, []llvm.Value{param}, "")
+		result = b.createZExtOrTrunc(result, b.intType)
+		b.CreateRet(result)
+		return true
+	case "Reverse", "Reverse8", "Reverse16", "Reverse32", "Reverse64",
+		"ReverseBytes", "ReverseBytes16", "ReverseBytes32", "ReverseBytes64":
+		b.createFunctionStart(true)
+		param := b.getValue(b.fn.Params[0], b.fn.Pos())
+		valueType := param.Type()
+		var intrinsicName string
+		if strings.HasPrefix(name, "ReverseBytes") {
+			intrinsicName = "llvm.bswap.i" + strconv.Itoa(valueType.IntTypeWidth())
+		} else { // Reverse
+			intrinsicName = "llvm.bitreverse.i" + strconv.Itoa(valueType.IntTypeWidth())
+		}
+		llvmFn := b.mod.NamedFunction(intrinsicName)
+		llvmFnType := llvm.FunctionType(valueType, []llvm.Type{valueType}, false)
+		if llvmFn.IsNil() {
+			llvmFn = llvm.AddFunction(b.mod, intrinsicName, llvmFnType)
+		}
+		result := b.createCall(llvmFnType, llvmFn, []llvm.Value{param}, "")
+		b.CreateRet(result)
+		return true
+	case "RotateLeft", "RotateLeft8", "RotateLeft16", "RotateLeft32", "RotateLeft64":
+		// Warning: the documentation says these functions must be constant time.
+		// I do not think LLVM guarantees this, but there's a good chance LLVM
+		// already recognized the rotate instruction so it probably won't get
+		// any _worse_ by implementing these rotate functions.
+		b.createFunctionStart(true)
+		x := b.getValue(b.fn.Params[0], b.fn.Pos())
+		k := b.getValue(b.fn.Params[1], b.fn.Pos())
+		valueType := x.Type()
+		intrinsicName := "llvm.fshl.i" + strconv.Itoa(valueType.IntTypeWidth())
+		llvmFn := b.mod.NamedFunction(intrinsicName)
+		llvmFnType := llvm.FunctionType(valueType, []llvm.Type{valueType, valueType, valueType}, false)
+		if llvmFn.IsNil() {
+			llvmFn = llvm.AddFunction(b.mod, intrinsicName, llvmFnType)
+		}
+		k = b.createZExtOrTrunc(k, valueType)
+		result := b.createCall(llvmFnType, llvmFn, []llvm.Value{x, x, k}, "")
+		b.CreateRet(result)
+		return true
+	default:
+		return false
+	}
+}

compiler/llvm.go

@@ -464,6 +464,19 @@ func (b *builder) readStackPointer() llvm.Value {
 	return b.CreateCall(stacksave.GlobalValueType(), stacksave, nil, "")
 }
 
+// createZExtOrTrunc lets the input value fit in the output type bits, by zero
+// extending or truncating the integer.
+func (b *builder) createZExtOrTrunc(value llvm.Value, t llvm.Type) llvm.Value {
+	valueBits := value.Type().IntTypeWidth()
+	resultBits := t.IntTypeWidth()
+	if valueBits > resultBits {
+		value = b.CreateTrunc(value, t, "")
+	} else if valueBits < resultBits {
+		value = b.CreateZExt(value, t, "")
+	}
+	return value
+}
+
 // Reverse a slice of bytes. From the wiki:
 // https://github.com/golang/go/wiki/SliceTricks#reversing
 func reverseBytes(buf []byte) {