Auto merge of #102570 - cjgillot:deagg-debuginfo, r=oli-obk

Perform simple scalar replacement of aggregates (SROA) MIR opt

This is a re-open of https://github.com/rust-lang/rust/pull/85796

I copied the debuginfo implementation (first commit) from `@eddyb's` own SROA PR.

This pass replaces plain field accesses by simple locals when possible.
To be eligible, the replaced locals:
- must not be enums or unions;
- must not be used whole;
- must not have their address taken.

The storage and deinit statements are duplicated on each created local.

cc `@tmiasko` who reviewed the former version of this PR.

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -39,6 +39,7 @@ use smallvec::SmallVec;
 use std::cell::OnceCell;
 use std::cell::RefCell;
 use std::iter;
+use std::ops::Range;
 
 mod create_scope_map;
 pub mod gdb;
@@ -163,12 +164,14 @@ impl<'ll> DebugInfoBuilderMethods for Builder<'_, 'll, '_> {
         variable_alloca: Self::Value,
         direct_offset: Size,
         indirect_offsets: &[Size],
+        fragment: Option<Range<Size>>,
     ) {
-        // Convert the direct and indirect offsets to address ops.
+        // Convert the direct and indirect offsets and fragment byte range to address ops.
         // FIXME(eddyb) use `const`s instead of getting the values via FFI,
         // the values should match the ones in the DWARF standard anyway.
         let op_deref = || unsafe { llvm::LLVMRustDIBuilderCreateOpDeref() };
         let op_plus_uconst = || unsafe { llvm::LLVMRustDIBuilderCreateOpPlusUconst() };
+        let op_llvm_fragment = || unsafe { llvm::LLVMRustDIBuilderCreateOpLLVMFragment() };
         let mut addr_ops = SmallVec::<[u64; 8]>::new();
 
         if direct_offset.bytes() > 0 {
@@ -182,6 +185,13 @@ impl<'ll> DebugInfoBuilderMethods for Builder<'_, 'll, '_> {
                 addr_ops.push(offset.bytes() as u64);
             }
         }
+        if let Some(fragment) = fragment {
+            // `DW_OP_LLVM_fragment` takes as arguments the fragment's
+            // offset and size, both of them in bits.
+            addr_ops.push(op_llvm_fragment());
+            addr_ops.push(fragment.start.bits() as u64);
+            addr_ops.push((fragment.end - fragment.start).bits() as u64);
+        }
 
         unsafe {
             // FIXME(eddyb) replace `llvm.dbg.declare` with `llvm.dbg.addr`.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2210,6 +2210,7 @@ extern "C" {
     ) -> &'a DILocation;
     pub fn LLVMRustDIBuilderCreateOpDeref() -> u64;
     pub fn LLVMRustDIBuilderCreateOpPlusUconst() -> u64;
+    pub fn LLVMRustDIBuilderCreateOpLLVMFragment() -> u64;
 
     #[allow(improper_ctypes)]
     pub fn LLVMRustWriteTypeToString(Type: &Type, s: &RustString);