Auto merge of #95685 - oxidecomputer:restore-static-dwarf, r=pnkfelix

Revert "Work around invalid DWARF bugs for fat LTO"

Since September, the toolchain has not been generating reliable DWARF
information for static variables when LTO is on. This has affected
projects in the embedded space where the use of LTO is typical. In our
case, it has kept us from bumping past the 2021-09-22 nightly toolchain
lest our debugger break. This has been a pretty dramatic regression for
people using debuggers and static variables. See #90357 for more info
and a repro case.

This commit is a mechanical revert of
d5de680e20def848751cb3c11e1182408112b1d3 from PR #89041, which caused
the issue. (Note on that PR that the commit's author has requested it be
reverted.)

I have locally verified that this fixes #90357 by restoring the
functionality of both the repro case I posted on that bug, and debugger
behavior on real programs. There do not appear to be test cases for this
in the toolchain; if I've missed them, point me at 'em and I'll update
them.

2 files changed, 8 insertions, 18 deletions

diff --git a/compiler/rustc_codegen_llvm/src/back/lto.rs b/compiler/rustc_codegen_llvm/src/back/lto.rs
index 38402e04313..be539499b56 100644
--- a/compiler/rustc_codegen_llvm/src/back/lto.rs
+++ b/compiler/rustc_codegen_llvm/src/back/lto.rs
@@ -325,20 +325,6 @@ fn fat_lto(
         drop(linker);
         save_temp_bitcode(cgcx, &module, "lto.input");
 
-        // Fat LTO also suffers from the invalid DWARF issue similar to Thin LTO.
-        // Here we rewrite all `DICompileUnit` pointers if there is only one `DICompileUnit`.
-        // This only works around the problem when codegen-units = 1.
-        // Refer to the comments in the `optimize_thin_module` function for more details.
-        let mut cu1 = ptr::null_mut();
-        let mut cu2 = ptr::null_mut();
-        unsafe { llvm::LLVMRustLTOGetDICompileUnit(llmod, &mut cu1, &mut cu2) };
-        if !cu2.is_null() {
-            let _timer =
-                cgcx.prof.generic_activity_with_arg("LLVM_fat_lto_patch_debuginfo", &*module.name);
-            unsafe { llvm::LLVMRustLTOPatchDICompileUnit(llmod, cu1) };
-            save_temp_bitcode(cgcx, &module, "fat-lto-after-patch");
-        }
-
         // Internalize everything below threshold to help strip out more modules and such.
         unsafe {
             let ptr = symbols_below_threshold.as_ptr();
@@ -769,7 +755,7 @@ pub unsafe fn optimize_thin_module(
         // an error.
         let mut cu1 = ptr::null_mut();
         let mut cu2 = ptr::null_mut();
-        llvm::LLVMRustLTOGetDICompileUnit(llmod, &mut cu1, &mut cu2);
+        llvm::LLVMRustThinLTOGetDICompileUnit(llmod, &mut cu1, &mut cu2);
         if !cu2.is_null() {
             let msg = "multiple source DICompileUnits found";
             return Err(write::llvm_err(&diag_handler, msg));
@@ -858,7 +844,7 @@ pub unsafe fn optimize_thin_module(
             let _timer = cgcx
                 .prof
                 .generic_activity_with_arg("LLVM_thin_lto_patch_debuginfo", thin_module.name());
-            llvm::LLVMRustLTOPatchDICompileUnit(llmod, cu1);
+            llvm::LLVMRustThinLTOPatchDICompileUnit(llmod, cu1);
             save_temp_bitcode(cgcx, &module, "thin-lto-after-patch");
         }
 
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 5ebc2d6139f..0ad65e5d99b 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -2508,8 +2508,12 @@ extern "C" {
         len: usize,
         out_len: &mut usize,
     ) -> *const u8;
-    pub fn LLVMRustLTOGetDICompileUnit(M: &Module, CU1: &mut *mut c_void, CU2: &mut *mut c_void);
-    pub fn LLVMRustLTOPatchDICompileUnit(M: &Module, CU: *mut c_void);
+    pub fn LLVMRustThinLTOGetDICompileUnit(
+        M: &Module,
+        CU1: &mut *mut c_void,
+        CU2: &mut *mut c_void,
+    );
+    pub fn LLVMRustThinLTOPatchDICompileUnit(M: &Module, CU: *mut c_void);
 
     pub fn LLVMRustLinkerNew(M: &Module) -> &mut Linker<'_>;
     pub fn LLVMRustLinkerAdd(