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add comment on uninitialized memory

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Folkert 2021-10-30 22:14:26 +02:00
parent 29e7853b85
commit 36496662d2
1 changed files with 17 additions and 0 deletions
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17
compiler/gen_llvm/src/llvm/build.rs
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@ -1461,6 +1461,23 @@ pub fn build_tag<'a, 'ctx, 'env>(
.struct_type(&[internal_type, tag_id_type.into()], false);
let result_alloca = env.builder.build_alloca(wrapper_type, "tag_opaque");
// Initialize all memory of the alloca. This _should_ not be required, but currently
// LLVM can access uninitialized memory after applying some optimizations. Hopefully
// we can in the future adjust code gen so this is no longer an issue.
//
// An example is
//
// main : Task.Task {} []
// main =
// when List.len [ Ok "foo", Err 42, Ok "spam" ] is
// n -> Task.putLine (Str.fromInt n)
//
// Here the decrement function of result must first check it's an Ok tag,
// then defers to string decrement, which must check is the string is small or large
//
// After inlining, those checks are combined. That means that even if the tag is Err,
// a check is done on the "string" to see if it is big or small, which will touch the
// uninitialized memory.
let all_zeros = wrapper_type.const_zero();
env.builder.build_store(result_alloca, all_zeros);