Merge pull request #354 from WebAssembly/jfbastien-patch-3

Limit immediate address and alignment

AstSemantics.md

@@ -119,9 +119,10 @@ reproduce their value operand, with no conversion applied.
 ### Addressing
 
 Each linear memory access operation also has an address operand and an immediate
-integer byte offset attribute. The infinite-precision sum of the address
-operand's value with the byte offset attribute's value is called the
-*effective address*, which is interpreted as an unsigned byte index.
+integer byte offset attribute. The immediate is the same type as the address'
+index. The infinite-precision sum of the address operand's value with the byte
+offset attribute's value is called the *effective address*, which is interpreted
+as an unsigned byte index.
 
 Linear memory accesses access the bytes starting at the location in the linear
 memory storage indexed by the effective address, and extending for the number
@@ -150,8 +151,9 @@ will add support for wasm64 and thus
 ### Alignment
 
 Each linear memory access operation also has an immediate positive integer power
-of 2 alignment attribute. An alignment value which is the same as the memory
-attribute size is considered to be a *natural* alignment.
+of 2 alignment attribute, which is the same type as the address' index. An
+alignment value which is the same as the memory attribute size is considered
+to be a *natural* alignment.
 
 The linear memory operation names listed above specify natural alignment.
 To specify unnatural alignment, the opcode name can be suffixed with `/n` for