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remove optimization when values move between pointers, it was unsound (see comment in source)

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Folkert 2022-03-26 18:50:47 +01:00
parent e6ae01384f
commit 75ab8afc2b
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1 changed files with 31 additions and 20 deletions
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51
compiler/gen_llvm/src/llvm/build.rs
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@ -2537,28 +2537,39 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
value,
layout
);
let value_ptr = value.into_pointer_value();
// We can only do this if the function itself writes data into this
// pointer. If the pointer is passed as an argument, then we must copy
// from one pointer to our destination pointer
if value_ptr.get_first_use().is_some() {
value_ptr.replace_all_uses_with(destination);
} else {
let size = env
.ptr_int()
.const_int(layout.stack_size(env.target_info) as u64, false);
// What we want to do here is
//
// let value_ptr = value.into_pointer_value();
// if false && value_ptr.get_first_use().is_some() {
// value_ptr.replace_all_uses_with(destination);
//
// In other words, if the source pointer is used,
// then we just subsitute the source for the input pointer, done.
//
// Only that does not work if the source is not written to.
// A simple example is the identity function
//
// A slightly more complex case that will also make the above not
// work is when the source pointer is only incremented, but not
// written to. Then there is a first_use, but it's still invalid to
// subsitute source with destination
//
// Hence, we explicitly memcpy source to destination, and rely on
// LLVM optimizing away any inefficiencies.
let size = env
.ptr_int()
.const_int(layout.stack_size(env.target_info) as u64, false);
env.builder
.build_memcpy(
destination,
align_bytes,
value.into_pointer_value(),
align_bytes,
size,
)
.unwrap();
}
env.builder
.build_memcpy(
destination,
align_bytes,
value.into_pointer_value(),
align_bytes,
size,
)
.unwrap();
}
} else {
env.builder.build_store(destination, value);