Rollup merge of #157127 - scottmcm:tweak-layout-of-alternative, r=nikic

cg_LLVM: Stop needing an alloca for volatile loads

This ended up also being reimplementing it to not use `load` of the `llvm_type`, since without doing that everything blew up horribly.  cc the zulip conversation [#t-opsem > Defining volatile splitting @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/136281-t-opsem/topic/Defining.20volatile.20splitting/near/597451615).

And while I'm here, improve the tests to check that the unaligned ones are actually unaligned, since `unaligned_volatile_load::<u8>` doesn't actually test anything.

r? @nikic
cc @RalfJung
MCP tracking issue rust-lang/rust#153250

Author: JonathanBrouwer

src/builder.rs

@@ -993,7 +993,8 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
         loaded_value.to_rvalue()
     }
 
-    fn volatile_load(&mut self, ty: Type<'gcc>, ptr: RValue<'gcc>) -> RValue<'gcc> {
+    fn volatile_load(&mut self, ty: Type<'gcc>, ptr: RValue<'gcc>, _: Align) -> RValue<'gcc> {
+        // FIXME(antoyo): set alignment.
         let ptr = self.context.new_cast(self.location, ptr, ty.make_volatile().make_pointer());
         // (FractalFir): We insert a local here, to ensure this volatile load can't move across
         // blocks.

src/intrinsic/mod.rs

@@ -5,7 +5,7 @@ mod simd;
 use std::iter;
 
 use gccjit::{ComparisonOp, Function, FunctionType, RValue, ToRValue, Type, UnaryOp};
-use rustc_abi::{BackendRepr, HasDataLayout, WrappingRange};
+use rustc_abi::{Align, BackendRepr, HasDataLayout, WrappingRange};
 use rustc_codegen_ssa::base::wants_msvc_seh;
 use rustc_codegen_ssa::common::IntPredicate;
 use rustc_codegen_ssa::errors::InvalidMonomorphization;
@@ -368,8 +368,9 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tc
 
             sym::volatile_load | sym::unaligned_volatile_load => {
                 let ptr = args[0].immediate();
-                let load = self.volatile_load(result.layout.gcc_type(self), ptr);
-                // FIXME(antoyo): set alignment.
+                let abi_align = result_layout.align.abi;
+                let ptr_align = if name == sym::volatile_load { abi_align } else { Align::ONE };
+                let load = self.volatile_load(result.layout.gcc_type(self), ptr, ptr_align);
                 if let BackendRepr::Scalar(scalar) = result.layout.backend_repr {
                     self.to_immediate_scalar(load, scalar)
                 } else {