From: Brendan Hansen <brendan.f.hansen@gmail.com>
Date: Mon, 9 Jan 2023 01:57:39 +0000 (-0600)
Subject: added document explaining issues
X-Git-Url: https://git.brendanfh.com/?a=commitdiff_plain;h=4485358724c5c89fda18ee712035b1d96f0d635a;p=onyx.git

added document explaining issues
---

diff --git a/docs/struct_asm_rewrite.md b/docs/struct_asm_rewrite.md
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+Rewriting Structure Assembly Code
+=================================
+
+Current in the Onyx code generator, structures r-values are represented
+in the execution stack as each of their individual primitive memebers.
+This solution has proved relatively easy to implement, but takes A LOT
+of extra instructions compared to the alternative.
+
+I would like to rewrite the way structure r-values are handled in the
+assembly to the following: all structure r-values are stored in the stack
+(or somewhere in memory, but probably the stack), and the execution stack
+simply contains a pointer to that address. This would allow for a structure
+load/store operation to simply be replaced by a COPY, instead of the potentially
+hundreds of instructions that it takes now.
+
+
+The largest issue that I have thought of so far with making this change
+is a flaw in how fixed-size arrays are being handled, which in turn would
+be a flaw with new structure r-values as well. This code example show it:
+
+        main :: () {
+            r := array_2(array_1(.[6, 7, 8, 9, 10]));
+            println(r);
+        }
+
+        array_1 :: (x: [5] u32) -> [5] u32 {
+            return .[ 1, 2, 3, 4, 5 ];
+        }
+
+        array_2 :: (x: [5] u32) -> [5] u32 {
+            y: [5] u32;
+            for^ y do *it = 1337;
+
+            printf("{} == {}\n", ^y, cast(^u32) x);
+
+            return x;
+        }
+
+Here, you would expect the print in `main` to be "[1, 2, 3, 4, 5]". However,
+because of a bug with where arrays are stored, you actually see "[1337, 1337, ...]".
+This bug is that because array literals are living in the stack space of the
+current function, returning then is the same thing as returning a pointer to
+a local variable. This works in the easiest case as show below, because the
+array is copied immediately into the result variable, which lives in `main`s
+stack, but not when anything more complicated is going on:
+        
+        main :: () {
+            result := array_1(.[]);
+            println(result);
+        }
+
+This bug is easy to get around when working with fixed-size arrays, which is
+something I do rarely anyway. However, structures are different story. Chaining
+calls that return structures is done every where in Onyx programs. Just take the
+`core.iter` package as an example.
+
+        main :: () {
+            for iter.as_iter(1 .. 20)
+                |> iter.map(x => x * 2)
+                |> iter.filter(x => x > 4)
+                |> iter.take(5) {
+
+                println(it);
+            }
+        }
+
+In this example, each of the return values of those functions is an `Iterator(i32)`,
+which is obviously a structure. In the new system, passing each result as a paramter
+to the next function would not work as shown with the array example above. A solution
+to this problem must be found before this refactor can be completed.
+