compiler: refactor slice related asserts

Move these asserts into compiler/asserts.go, to keep them together.

The make([]T) asserts aren't moved yet because that code is (still!)
quite ugly and in need of some clean up.

compiler/asserts.go

@@ -4,9 +4,82 @@ package compiler
 // required by the Go programming language.
 
 import (
+	"go/types"
+
 	"tinygo.org/x/go-llvm"
 )
 
+// emitLookupBoundsCheck emits a bounds check before doing a lookup into a
+// slice. This is required by the Go language spec: an index out of bounds must
+// cause a panic.
+func (c *Compiler) emitLookupBoundsCheck(frame *Frame, arrayLen, index llvm.Value, indexType types.Type) {
+	if frame.fn.IsNoBounds() {
+		// The //go:nobounds pragma was added to the function to avoid bounds
+		// checking.
+		return
+	}
+
+	// Sometimes, the index can be e.g. an uint8 or int8, and we have to
+	// correctly extend that type.
+	if index.Type().IntTypeWidth() < arrayLen.Type().IntTypeWidth() {
+		if indexType.(*types.Basic).Info()&types.IsUnsigned == 0 {
+			index = c.builder.CreateZExt(index, arrayLen.Type(), "")
+		} else {
+			index = c.builder.CreateSExt(index, arrayLen.Type(), "")
+		}
+	}
+
+	// Optimize away trivial cases.
+	// LLVM would do this anyway with interprocedural optimizations, but it
+	// helps to see cases where bounds check elimination would really help.
+	if index.IsConstant() && arrayLen.IsConstant() && !arrayLen.IsUndef() {
+		index := index.SExtValue()
+		arrayLen := arrayLen.SExtValue()
+		if index >= 0 && index < arrayLen {
+			return
+		}
+	}
+
+	if index.Type().IntTypeWidth() > c.intType.IntTypeWidth() {
+		// Index is too big for the regular bounds check. Use the one for int64.
+		c.createRuntimeCall("lookupBoundsCheckLong", []llvm.Value{arrayLen, index}, "")
+	} else {
+		c.createRuntimeCall("lookupBoundsCheck", []llvm.Value{arrayLen, index}, "")
+	}
+}
+
+// emitSliceBoundsCheck emits a bounds check before a slicing operation to make
+// sure it is within bounds.
+func (c *Compiler) emitSliceBoundsCheck(frame *Frame, capacity, low, high llvm.Value, lowType, highType *types.Basic) {
+	if frame.fn.IsNoBounds() {
+		// The //go:nobounds pragma was added to the function to avoid bounds
+		// checking.
+		return
+	}
+
+	uintptrWidth := c.uintptrType.IntTypeWidth()
+	if low.Type().IntTypeWidth() > uintptrWidth || high.Type().IntTypeWidth() > uintptrWidth {
+		if low.Type().IntTypeWidth() < 64 {
+			if lowType.Info()&types.IsUnsigned != 0 {
+				low = c.builder.CreateZExt(low, c.ctx.Int64Type(), "")
+			} else {
+				low = c.builder.CreateSExt(low, c.ctx.Int64Type(), "")
+			}
+		}
+		if high.Type().IntTypeWidth() < 64 {
+			if highType.Info()&types.IsUnsigned != 0 {
+				high = c.builder.CreateZExt(high, c.ctx.Int64Type(), "")
+			} else {
+				high = c.builder.CreateSExt(high, c.ctx.Int64Type(), "")
+			}
+		}
+		// TODO: 32-bit or even 16-bit slice bounds checks for 8-bit platforms
+		c.createRuntimeCall("sliceBoundsCheck64", []llvm.Value{capacity, low, high}, "")
+	} else {
+		c.createRuntimeCall("sliceBoundsCheck", []llvm.Value{capacity, low, high}, "")
+	}
+}
+
 // emitNilCheck checks whether the given pointer is nil, and panics if it is. It
 // has no effect in well-behaved programs, but makes sure no uncaught nil
 // pointer dereferences exist in valid Go code.