diff options
| -rw-r--r-- | doc/il.txt | 50 |
1 files changed, 47 insertions, 3 deletions
diff --git a/doc/il.txt b/doc/il.txt index 08dc2a6..a5b796b 100644 --- a/doc/il.txt +++ b/doc/il.txt @@ -26,7 +26,7 @@ 6. <@ Regular Instructions > * <@ Arithmetic > * <@ Memory > - * Comparisons + * <@ Comparisons > 7. Special Instructions * Conversions * Casts @@ -492,7 +492,6 @@ For example, consider the type string `wl(F)`, it mentions that the instruction has only one argument and that if the return type used is long, the argument must be of type double. - ~ Arithmetic ~~~~~~~~~~~~ @@ -525,7 +524,8 @@ return type used is long, the argument must be of type double. * `stores` -- `(s,m)` * `storel` -- `(l,m)` * `storew` -- `(w,m)` - * `storeh`, `storeb` -- `(w,m)` + * `storeh` -- `(w,m)` + * `storeb` -- `(w,m)` Store instructions exist to store a value of any base type and any extended type. Since halfwords and bytes are not @@ -542,3 +542,47 @@ return type used is long, the argument must be of type double. * `loadsw`, `loadzw` -- `I(mm)` * `loadsh`, `loadzh` -- `I(mm)` * `loadsb`, `loadzb` -- `I(mm)` + + For types smaller than long, two variants of the load + instruction is available: one will sign extend the value + loaded, while the other will zero extend it. Remark that + all loads smaller than long can load to either a long or + a word. + + The two instructions `loadsw` and `loadzw` have the same + effect when they are used to define a word temporary. + A `loadw` instruction is provided as syntactic sugar for + `loadsw` to make explicit that the extension mechanism + used is irrelevant. + + * Stack allocation. + + * `alloc4` -- `m(l)` + * `alloc8` -- `m(l)` + * `alloc16` -- `m(l)` + + These instructions allocate a chunk of memory on the + stack. The number ending the instruction name is the + alignment required for the allocated slot. QBE will + make sure that the returned address is a multiple of + that alignment value. + + Stack allocation instructions are used, for example, + when compiling the C local variables, because their + address can be taken. When compiling Fortran, + temporaries can be used directly instead, because + it is illegal to take the address of a variable. + +The following example makes use some of the memory +instructions. Pointers are stored in long temporaries. + + %A0 =l alloc4 8 # stack allocate an array A of 2 words + %A1 =l add %A0, 4 + storew 43, %A0 # A[0] <- 43 + storew 255, %A1 # A[1] <- 255 + %v1 =w loadw %A0 # %v1 <- A[0] as word + %v2 =w loadsb %A1 # %v2 <- A[1] as signed byte + %v3 =w add %v1, %v2 # %v3 is 42 here + +~ Comparisons +~~~~~~~~~~~~~ |