Auto merge of rust-lang#133194 - khuey:master, r=jieyouxu

Drop debug info instead of panicking if we exceed LLVM's capability to represent it

Recapping a bit of history here:

In rust-lang#128861 I made debug info correctly represent parameters to inline functions by removing a fake lexical block that had been inserted to suppress LLVM assertions and by deduplicating those parameters.

LLVM, however, expects to see a single parameter _with distinct locations_, particularly distinct inlinedAt values on the DILocations. This generally worked because no matter how deep the chain of inlines it takes two different call sites in the original function to result in the same function being present multiple times, and a function call requires a non-zero number of characters, but macros threw a wrench in that in rust-lang#131944. At the time I thought the issue there was limited to proc-macros, where an arbitrary amount of code can be generated at a single point in the source text.

In rust-lang#132613 I added discriminators to DILocations that would otherwise be the same to repair rust-lang#131944[^1]. This works, but LLVM's capacity for discriminators is not infinite (LLVM actually only allocates 12 bits for this internally). At the time I thought it would be very rare for anyone to hit the limit, but rust-lang#132900 proved me wrong. In the relatively-minimized test case it also became clear to me that the issue affects regular macros too, because the call to the inlined function will (without collapse_debuginfo on the macro) be attributed to the (repeated, if the macro is used more than once) textual callsite in the macro definition.

This PR fixes the panic by dropping debug info when we exceed LLVM's maximum discriminator value. There's also a preceding commit for a related but distinct issue: macros that use collapse_debuginfo should in fact have their inlinedAts collapsed to the macro callsite and thus not need discriminators at all (and not panic/warn accordingly when the discriminator limit is exhausted).

Fixes rust-lang#132900

r? `@jieyouxu`

[^1]: Editor's note: `fix` is a magic keyword in PR description that apparently will close the linked issue (it's closed already in this case, but still).

compiler/rustc_codegen_gcc/src/debuginfo.rs

@@ -113,15 +113,15 @@ fn make_mir_scope<'gcc, 'tcx>(
     let scope_data = &mir.source_scopes[scope];
     let parent_scope = if let Some(parent) = scope_data.parent_scope {
         make_mir_scope(cx, _instance, mir, variables, debug_context, instantiated, parent);
-        debug_context.scopes[parent]
+        debug_context.scopes[parent].unwrap()
     } else {
         // The root is the function itself.
         let file = cx.sess().source_map().lookup_source_file(mir.span.lo());
-        debug_context.scopes[scope] = DebugScope {
+        debug_context.scopes[scope] = Some(DebugScope {
             file_start_pos: file.start_pos,
             file_end_pos: file.end_position(),
-            ..debug_context.scopes[scope]
-        };
+            ..debug_context.scopes[scope].unwrap()
+        });
         instantiated.insert(scope);
         return;
     };
@@ -130,7 +130,7 @@ fn make_mir_scope<'gcc, 'tcx>(
         if !vars.contains(scope) && scope_data.inlined.is_none() {
             // Do not create a DIScope if there are no variables defined in this
             // MIR `SourceScope`, and it's not `inlined`, to avoid debuginfo bloat.
-            debug_context.scopes[scope] = parent_scope;
+            debug_context.scopes[scope] = Some(parent_scope);
             instantiated.insert(scope);
             return;
         }
@@ -157,12 +157,12 @@ fn make_mir_scope<'gcc, 'tcx>(
     // TODO(tempdragon): dbg_scope: Add support for scope extension here.
     inlined_at.or(p_inlined_at);
 
-    debug_context.scopes[scope] = DebugScope {
+    debug_context.scopes[scope] = Some(DebugScope {
         dbg_scope,
         inlined_at,
         file_start_pos: loc.file.start_pos,
         file_end_pos: loc.file.end_position(),
-    };
+    });
     instantiated.insert(scope);
 }
 
@@ -232,12 +232,12 @@ impl<'gcc, 'tcx> DebugInfoCodegenMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         }
 
         // Initialize fn debug context (including scopes).
-        let empty_scope = DebugScope {
+        let empty_scope = Some(DebugScope {
             dbg_scope: self.dbg_scope_fn(instance, fn_abi, Some(llfn)),
             inlined_at: None,
             file_start_pos: BytePos(0),
             file_end_pos: BytePos(0),
-        };
+        });
         let mut fn_debug_context = FunctionDebugContext {
             scopes: IndexVec::from_elem(empty_scope, mir.source_scopes.as_slice()),
             inlined_function_scopes: Default::default(),

compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs

@@ -9,7 +9,7 @@ use rustc_middle::mir::{Body, SourceScope};
 use rustc_middle::ty::layout::{FnAbiOf, HasTypingEnv};
 use rustc_middle::ty::{self, Instance};
 use rustc_session::config::DebugInfo;
-use rustc_span::BytePos;
+use rustc_span::{BytePos, hygiene};
 
 use super::metadata::file_metadata;
 use super::utils::DIB;
@@ -85,15 +85,23 @@ fn make_mir_scope<'ll, 'tcx>(
             discriminators,
             parent,
         );
-        debug_context.scopes[parent]
+        if let Some(parent_scope) = debug_context.scopes[parent] {
+            parent_scope
+        } else {
+            // If the parent scope could not be represented then no children
+            // can be either.
+            debug_context.scopes[scope] = None;
+            instantiated.insert(scope);
+            return;
+        }
     } else {
         // The root is the function itself.
         let file = cx.sess().source_map().lookup_source_file(mir.span.lo());
-        debug_context.scopes[scope] = DebugScope {
+        debug_context.scopes[scope] = Some(DebugScope {
             file_start_pos: file.start_pos,
             file_end_pos: file.end_position(),
-            ..debug_context.scopes[scope]
-        };
+            ..debug_context.scopes[scope].unwrap()
+        });
         instantiated.insert(scope);
         return;
     };
@@ -104,7 +112,7 @@ fn make_mir_scope<'ll, 'tcx>(
     {
         // Do not create a DIScope if there are no variables defined in this
         // MIR `SourceScope`, and it's not `inlined`, to avoid debuginfo bloat.
-        debug_context.scopes[scope] = parent_scope;
+        debug_context.scopes[scope] = Some(parent_scope);
         instantiated.insert(scope);
         return;
     }
@@ -137,22 +145,28 @@ fn make_mir_scope<'ll, 'tcx>(
         },
     };
 
-    let inlined_at = scope_data.inlined.map(|(_, callsite_span)| {
-        // FIXME(eddyb) this doesn't account for the macro-related
-        // `Span` fixups that `rustc_codegen_ssa::mir::debuginfo` does.
+    let mut debug_scope = Some(DebugScope {
+        dbg_scope,
+        inlined_at: parent_scope.inlined_at,
+        file_start_pos: loc.file.start_pos,
+        file_end_pos: loc.file.end_position(),
+    });
+
+    if let Some((_, callsite_span)) = scope_data.inlined {
+        let callsite_span = hygiene::walk_chain_collapsed(callsite_span, mir.span);
         let callsite_scope = parent_scope.adjust_dbg_scope_for_span(cx, callsite_span);
         let loc = cx.dbg_loc(callsite_scope, parent_scope.inlined_at, callsite_span);
 
         // NB: In order to produce proper debug info for variables (particularly
-        // arguments) in multiply-inline functions, LLVM expects to see a single
+        // arguments) in multiply-inlined functions, LLVM expects to see a single
         // DILocalVariable with multiple different DILocations in the IR. While
         // the source information for each DILocation would be identical, their
         // inlinedAt attributes will be unique to the particular callsite.
         //
         // We generate DILocations here based on the callsite's location in the
         // source code. A single location in the source code usually can't
         // produce multiple distinct calls so this mostly works, until
-        // proc-macros get involved. A proc-macro can generate multiple calls
+        // macros get involved. A macro can generate multiple calls
         // at the same span, which breaks the assumption that we're going to
         // produce a unique DILocation for every scope we process here. We
         // have to explicitly add discriminators if we see inlines into the
@@ -161,24 +175,29 @@ fn make_mir_scope<'ll, 'tcx>(
         // Note further that we can't key this hashtable on the span itself,
         // because these spans could have distinct SyntaxContexts. We have
         // to key on exactly what we're giving to LLVM.
-        match discriminators.entry(callsite_span.lo()) {
+        let inlined_at = match discriminators.entry(callsite_span.lo()) {
             Entry::Occupied(mut o) => {
                 *o.get_mut() += 1;
                 unsafe { llvm::LLVMRustDILocationCloneWithBaseDiscriminator(loc, *o.get()) }
-                    .expect("Failed to encode discriminator in DILocation")
             }
             Entry::Vacant(v) => {
                 v.insert(0);
-                loc
+                Some(loc)
+            }
+        };
+        match inlined_at {
+            Some(inlined_at) => {
+                debug_scope.as_mut().unwrap().inlined_at = Some(inlined_at);
+            }
+            None => {
+                // LLVM has a maximum discriminator that it can encode (currently
+                // it uses 12 bits for 4096 possible values). If we exceed that
+                // there is little we can do but drop the debug info.
+                debug_scope = None;
             }
         }
-    });
+    }
 
-    debug_context.scopes[scope] = DebugScope {
-        dbg_scope,
-        inlined_at: inlined_at.or(parent_scope.inlined_at),
-        file_start_pos: loc.file.start_pos,
-        file_end_pos: loc.file.end_position(),
-    };
+    debug_context.scopes[scope] = debug_scope;
     instantiated.insert(scope);
 }

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -294,12 +294,12 @@ impl<'ll, 'tcx> DebugInfoCodegenMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         }
 
         // Initialize fn debug context (including scopes).
-        let empty_scope = DebugScope {
+        let empty_scope = Some(DebugScope {
             dbg_scope: self.dbg_scope_fn(instance, fn_abi, Some(llfn)),
             inlined_at: None,
             file_start_pos: BytePos(0),
             file_end_pos: BytePos(0),
-        };
+        });
         let mut fn_debug_context = FunctionDebugContext {
             scopes: IndexVec::from_elem(empty_scope, &mir.source_scopes),
             inlined_function_scopes: Default::default(),

compiler/rustc_codegen_ssa/src/mir/debuginfo.rs

@@ -20,7 +20,9 @@ use crate::traits::*;
 
 pub struct FunctionDebugContext<'tcx, S, L> {
     /// Maps from source code to the corresponding debug info scope.
-    pub scopes: IndexVec<mir::SourceScope, DebugScope<S, L>>,
+    /// May be None if the backend is not capable of representing the scope for
+    /// some reason.
+    pub scopes: IndexVec<mir::SourceScope, Option<DebugScope<S, L>>>,
 
     /// Maps from an inlined function to its debug info declaration.
     pub inlined_function_scopes: FxHashMap<Instance<'tcx>, S>,
@@ -231,7 +233,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         &self,
         source_info: mir::SourceInfo,
     ) -> Option<(Bx::DIScope, Option<Bx::DILocation>, Span)> {
-        let scope = &self.debug_context.as_ref()?.scopes[source_info.scope];
+        let scope = &self.debug_context.as_ref()?.scopes[source_info.scope]?;
         let span = hygiene::walk_chain_collapsed(source_info.span, self.mir.span);
         Some((scope.adjust_dbg_scope_for_span(self.cx, span), scope.inlined_at, span))
     }