Rollup merge of #117671 - kjetilkjeka:nvptx_c_abi_avoid_direct, r=davidtwco

NVPTX: Avoid PassMode::Direct for args in C abi

Fixes #117480

I must admit that I'm confused about `PassMode` altogether, is there a good sum-up threads for this anywhere? I'm especially confused about how "indirect" and "byval" goes together. To me it seems like "indirect" basically means "use a indirection through a pointer", while "byval" basically means "do not use indirection through a pointer".

The return used to keep `PassMode::Direct` for small aggregates. It turns out that `make_indirect` messes up the tests and one way to fix it is to keep `PassMode::Direct` for all aggregates. I have mostly seen this PassMode mentioned for args. Is it also a problem for returns? When experimenting with `byval` as an alternative i ran into [this assert](https://github.com/rust-lang/rust/blob/61a3eea8043cc1c7a09c2adda884e27ffa8a1172/compiler/rustc_codegen_llvm/src/abi.rs#L463C22-L463C22)

I have added tests for the same kind of types that is already tested for the "ptx-kernel" abi. The tests cannot be enabled until something like #117458 is completed and merged.

CC: ``@RalfJung`` since you seem to be the expert on this and have already helped me out tremendously

CC: ``@RDambrosio016`` in case this influence your work on `rustc_codegen_nvvm`

``@rustbot`` label +O-NVPTX

1 files changed, 4 insertions, 5 deletions

diff --git a/compiler/rustc_target/src/abi/call/nvptx64.rs b/compiler/rustc_target/src/abi/call/nvptx64.rs
index f85fa2419f0..c6b6f66e6be 100644
--- a/compiler/rustc_target/src/abi/call/nvptx64.rs
+++ b/compiler/rustc_target/src/abi/call/nvptx64.rs
@@ -11,11 +11,10 @@ fn classify_ret<Ty>(ret: &mut ArgAbi<'_, Ty>) {
 }
 
 fn classify_arg<Ty>(arg: &mut ArgAbi<'_, Ty>) {
-    if arg.layout.is_aggregate() && arg.layout.size.bits() > 64 {
-        arg.make_indirect();
-    } else {
-        // FIXME: this is wrong! Need to decide which ABI we really want here.
-        arg.make_direct_deprecated();
+    if arg.layout.is_aggregate() {
+        arg.make_indirect_byval(None);
+    } else if arg.layout.size.bits() < 32 {
+        arg.extend_integer_width_to(32);
     }
 }