compiler: fix escapes due to nil checks

Some tests get bigger, most get smaller. However, all tested driver
examples get smaller in size showing that this is a good change in the
real world.

compiler/asserts.go

@@ -115,8 +115,23 @@ func (c *Compiler) emitNilCheck(frame *Frame, ptr llvm.Value, blockPrefix string
 	frame.blockExits[frame.currentBlock] = nextBlock // adjust outgoing block for phi nodes
 
 	// Compare against nil.
-	nilptr := llvm.ConstPointerNull(ptr.Type())
-	isnil := c.builder.CreateICmp(llvm.IntEQ, ptr, nilptr, "")
+	var isnil llvm.Value
+	if ptr.Type().PointerAddressSpace() == 0 {
+		// Do the nil check using the isnil builtin, which marks the parameter
+		// as nocapture.
+		// The reason it has to go through a builtin, is that a regular icmp
+		// instruction may capture the pointer in LLVM semantics, see
+		// https://reviews.llvm.org/D60047 for details. Pointer capturing
+		// unfortunately breaks escape analysis, so we use this trick to let the
+		// functionattr pass know that this pointer doesn't really escape.
+		ptr = c.builder.CreateBitCast(ptr, c.i8ptrType, "")
+		isnil = c.createRuntimeCall("isnil", []llvm.Value{ptr}, "")
+	} else {
+		// Do the nil check using a regular icmp. This can happen with function
+		// pointers on AVR, which don't benefit from escape analysis anyway.
+		nilptr := llvm.ConstPointerNull(ptr.Type())
+		isnil = c.builder.CreateICmp(llvm.IntEQ, ptr, nilptr, "")
+	}
 	c.builder.CreateCondBr(isnil, faultBlock, nextBlock)
 
 	// Fail: this is a nil pointer, exit with a panic.

compiler/compiler.go

@@ -379,6 +379,11 @@ func (c *Compiler) Compile(mainPath string) error {
 		c.mod.NamedFunction("runtime.alloc").AddAttributeAtIndex(0, attr)
 	}
 
+	// See emitNilCheck in asserts.go.
+	attrKind := llvm.AttributeKindID("nocapture")
+	attr := c.ctx.CreateEnumAttribute(attrKind, 0)
+	c.mod.NamedFunction("runtime.isnil").AddAttributeAtIndex(1, attr)
+
 	// see: https://reviews.llvm.org/D18355
 	if c.Debug {
 		c.mod.AddNamedMetadataOperand("llvm.module.flags",

compiler/optimizer.go

@@ -53,6 +53,22 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) erro
 		c.OptimizeAllocs()
 		c.OptimizeStringToBytes()
 
+		// Lower runtime.isnil calls to regular nil comparisons.
+		isnil := c.mod.NamedFunction("runtime.isnil")
+		if !isnil.IsNil() {
+			for _, use := range getUses(isnil) {
+				c.builder.SetInsertPointBefore(use)
+				ptr := use.Operand(0)
+				if !ptr.IsABitCastInst().IsNil() {
+					ptr = ptr.Operand(0)
+				}
+				nilptr := llvm.ConstPointerNull(ptr.Type())
+				icmp := c.builder.CreateICmp(llvm.IntEQ, ptr, nilptr, "")
+				use.ReplaceAllUsesWith(icmp)
+				use.EraseFromParentAsInstruction()
+			}
+		}
+
 		err := c.LowerGoroutines()
 		if err != nil {
 			return err

src/runtime/panic.go

@@ -27,6 +27,14 @@ func _recover() interface{} {
 	return nil
 }
 
+// See emitNilCheck in compiler/asserts.go.
+// This function is a dummy function that has its first and only parameter
+// marked 'nocapture' to work around a limitation in LLVM: a regular pointer
+// comparison captures the pointer.
+func isnil(ptr *uint8) bool {
+	return ptr == nil
+}
+
 // Panic when trying to dereference a nil pointer.
 func nilpanic() {
 	runtimePanic("nil pointer dereference")