diff --git a/src/librustc_target/asm/nvptx.rs b/src/librustc_target/asm/nvptx.rs
index cba069cd0542..43d16ae0f5d1 100644
--- a/src/librustc_target/asm/nvptx.rs
+++ b/src/librustc_target/asm/nvptx.rs
@@ -43,5 +43,7 @@ impl NvptxInlineAsmRegClass {
 }
 
 def_regs! {
+    // Registers in PTX are declared in the assembly.
+    // There are no predefined registers that one can use.
     Nvptx NvptxInlineAsmReg NvptxInlineAsmRegClass {}
 }