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Diffstat (limited to 'doc/il.txt')
| -rw-r--r-- | doc/il.txt | 31 |
1 files changed, 19 insertions, 12 deletions
diff --git a/doc/il.txt b/doc/il.txt index 4684296..a80ee22 100644 --- a/doc/il.txt +++ b/doc/il.txt @@ -116,23 +116,28 @@ starting with the sigil `?`. BASETY := 'w' | 'l' | 's' | 'd' # Base types EXTTY := BASETY | 'h' | 'b' # Extended types -We makes very minimal use of types. The types used are only -what is necessary for unambiguous compilation to machine -code and C interfacing. +The IL makes very minimal use of types. By design, the types +used are restricted to what is necessary for unambiguous +compilation to machine code and C interfacing. Unlike LLVM, +QBE is not using types as a mean to safety, they are only +here for semantics purposes. The four base types are `w` (word), `l` (long), `s` (single), and `d` (double), they stand respectively for 32 bits and 64 bits integers, and 32 bits and 64 bits floating points. -Temporaries in the IL can only have a basic type. +There are no pointer types available, pointers are typed +by an integer type sufficiently wide to represent all memory +addresses (e.g. `l` on x64). Temporaries in the IL can only +have a basic type. Extended types contain base types and add `h` (half word) and `b` (byte), respectively for 16 bits and 8 bits integers. They are used in <@ Aggregate Types> and <@ Data> definitions. -The IL also provides user-defined aggregate types, these are -used for C interfacing. The syntax used to designate them is -`:foo`. Details about their definition are given in the -<@ Aggregate Types > section. +For C interfacing, the IL also provides user-defined aggregate +types. The syntax used to designate them is `:foo`. Details +about their definition are given in the <@ Aggregate Types > +section. ~ Subtyping ~~~~~~~~~~~ @@ -142,10 +147,12 @@ Any value of type `l` can be used in a `w` context. When that happens only the 32 least significant bits of the word value are used. -Note that a long value must not be used in word context. -The rationale is that the 32 high bits of the extended long -value could very well be zeroes or the result of a sign -extension of the word. +Make note that it is the inverse of the usual subtyping on +integers (in C, we can safely use an `int` where a `long` +is expected). A long value cannot be used in word context. +The rationale is that a word can be signed or unsigned, so +extending it to a long can be done in two ways, either +by zero-extension, or by sign-extension. - 3. Immediate Constants ------------------------ |