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rustc: Move a comment to the right spot in trans

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I believe this comment here is mostly talking about the `ptrcast` function call
below, so move the comment down to that block.
This commit is contained in:
Alex Crichton 2017-09-13 15:04:51 -07:00
parent afb85cfd33
commit 19727c84dd
1 changed files with 23 additions and 24 deletions
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47
src/librustc_trans/callee.rs
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@ -53,35 +53,34 @@ pub fn get_fn<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
let sym = tcx.symbol_name(instance);
debug!("get_fn({:?}: {:?}) => {}", instance, fn_ty, sym);
// This is subtle and surprising, but sometimes we have to bitcast
// the resulting fn pointer. The reason has to do with external
// functions. If you have two crates that both bind the same C
// library, they may not use precisely the same types: for
// example, they will probably each declare their own structs,
// which are distinct types from LLVM's point of view (nominal
// types).
//
// Now, if those two crates are linked into an application, and
// they contain inlined code, you can wind up with a situation
// where both of those functions wind up being loaded into this
// application simultaneously. In that case, the same function
// (from LLVM's point of view) requires two types. But of course
// LLVM won't allow one function to have two types.
//
// What we currently do, therefore, is declare the function with
// one of the two types (whichever happens to come first) and then
// bitcast as needed when the function is referenced to make sure
// it has the type we expect.
//
// This can occur on either a crate-local or crate-external
// reference. It also occurs when testing libcore and in some
// other weird situations. Annoying.
// Create a fn pointer with the substituted signature.
let fn_ptr_ty = tcx.mk_fn_ptr(common::ty_fn_sig(ccx, fn_ty));
let llptrty = type_of::type_of(ccx, fn_ptr_ty);
let llfn = if let Some(llfn) = declare::get_declared_value(ccx, &sym) {
// This is subtle and surprising, but sometimes we have to bitcast
// the resulting fn pointer. The reason has to do with external
// functions. If you have two crates that both bind the same C
// library, they may not use precisely the same types: for
// example, they will probably each declare their own structs,
// which are distinct types from LLVM's point of view (nominal
// types).
//
// Now, if those two crates are linked into an application, and
// they contain inlined code, you can wind up with a situation
// where both of those functions wind up being loaded into this
// application simultaneously. In that case, the same function
// (from LLVM's point of view) requires two types. But of course
// LLVM won't allow one function to have two types.
//
// What we currently do, therefore, is declare the function with
// one of the two types (whichever happens to come first) and then
// bitcast as needed when the function is referenced to make sure
// it has the type we expect.
//
// This can occur on either a crate-local or crate-external
// reference. It also occurs when testing libcore and in some
// other weird situations. Annoying.
if common::val_ty(llfn) != llptrty {
debug!("get_fn: casting {:?} to {:?}", llfn, llptrty);
consts::ptrcast(llfn, llptrty)