Make the CFG metadata computation conditional on a flag.

cranelift/codegen/meta/src/shared/settings.rs

@@ -245,6 +245,21 @@ pub(crate) fn define() -> SettingGroup {
         true,
     );
 
+    settings.add_bool(
+        "machine_code_cfg_info",
+        "Generate CFG metadata for machine code.",
+        r#"
+            This increases metadata size and compile time, but allows for the
+            embedder to more easily post-process or analyze the generated
+            machine code. It provides code offsets for the start of each
+            basic block in the generated machine code, and a list of CFG
+            edges (with blocks identified by start offsets) between them.
+            This is useful for, e.g., machine-code analyses that verify certain
+            properties of the generated code.
+        "#,
+        false,
+    );
+
     // BaldrMonkey requires that not-yet-relocated function addresses be encoded
     // as all-ones bitpatterns.
     settings.add_bool(

cranelift/codegen/src/machinst/mod.rs

@@ -341,10 +341,16 @@ pub struct MachCompileResult {
     /// Debug info: stackslots to stack pointer offsets.
     pub stackslot_offsets: PrimaryMap<StackSlot, u32>,
     /// Basic-block layout info: block start offsets.
+    ///
+    /// This info is generated only if the `machine_code_cfg_info`
+    /// flag is set.
     pub bb_starts: Vec<CodeOffset>,
     /// Basic-block layout info: block edges. Each edge is `(from,
     /// to)`, where `from` and `to` are basic-block start offsets of
     /// the respective blocks.
+    ///
+    /// This info is generated only if the `machine_code_cfg_info`
+    /// flag is set.
     pub bb_edges: Vec<(CodeOffset, CodeOffset)>,
 }
 

cranelift/codegen/src/machinst/vcode.rs

@@ -486,6 +486,7 @@ impl<I: VCodeInst> VCode<I> {
         let _tt = timing::vcode_emit();
         let mut buffer = MachBuffer::new();
         let mut state = I::State::new(&*self.abi);
+        let cfg_metadata = self.flags().machine_code_cfg_info();
         let mut bb_starts: Vec<Option<CodeOffset>> = vec![];
 
         // The first M MachLabels are reserved for block indices, the next N MachLabels for
@@ -513,19 +514,21 @@ impl<I: VCodeInst> VCode<I> {
             buffer.bind_label(MachLabel::from_block(block));
             label_insn_iix[block as usize] = start;
 
-            // Track BB starts. If we have backed up due to MachBuffer
+            if cfg_metadata {
-            // branch opts, note that the removed blocks were removed.
+                // Track BB starts. If we have backed up due to MachBuffer
-            let cur_offset = buffer.cur_offset();
+                // branch opts, note that the removed blocks were removed.
-            if last_offset.is_some() && cur_offset <= last_offset.unwrap() {
+                let cur_offset = buffer.cur_offset();
-                for i in (0..bb_starts.len()).rev() {
+                if last_offset.is_some() && cur_offset <= last_offset.unwrap() {
-                    if bb_starts[i].is_some() && cur_offset > bb_starts[i].unwrap() {
+                    for i in (0..bb_starts.len()).rev() {
-                        break;
+                        if bb_starts[i].is_some() && cur_offset > bb_starts[i].unwrap() {
+                            break;
+                        }
+                        bb_starts[i] = None;
                     }
-                    bb_starts[i] = None;
                 }
+                bb_starts.push(Some(cur_offset));
+                last_offset = Some(cur_offset);
             }
-            bb_starts.push(Some(cur_offset));
-            last_offset = Some(cur_offset);
 
             for iix in start..end {
                 let srcloc = self.srclocs[iix as usize];
@@ -606,19 +609,21 @@ impl<I: VCodeInst> VCode<I> {
         // Create `bb_edges` and final (filtered) `bb_starts`.
         let mut final_bb_starts = vec![];
         let mut bb_edges = vec![];
-        for block in 0..self.num_blocks() {
+        if cfg_metadata {
-            if bb_starts[block].is_none() {
+            for block in 0..self.num_blocks() {
-                // Block was deleted by MachBuffer; skip.
+                if bb_starts[block].is_none() {
-                continue;
+                    // Block was deleted by MachBuffer; skip.
-            }
+                    continue;
-            let from = bb_starts[block].unwrap();
+                }
+                let from = bb_starts[block].unwrap();
 
-            final_bb_starts.push(from);
+                final_bb_starts.push(from);
-            // Resolve each `succ` label and add edges.
+                // Resolve each `succ` label and add edges.
-            let succs = self.block_succs(BlockIx::new(block as u32));
+                let succs = self.block_succs(BlockIx::new(block as u32));
-            for succ in succs.iter() {
+                for succ in succs.iter() {
-                let to = buffer.resolve_label_offset(MachLabel::from_block(succ.get()));
+                    let to = buffer.resolve_label_offset(MachLabel::from_block(succ.get()));
-                bb_edges.push((from, to));
+                    bb_edges.push((from, to));
+                }
             }
         }
 

cranelift/codegen/src/settings.rs

@@ -511,6 +511,7 @@ enable_atomics = true
 enable_safepoints = false
 enable_llvm_abi_extensions = false
 unwind_info = true
+machine_code_cfg_info = false
 emit_all_ones_funcaddrs = false
 enable_probestack = true
 probestack_func_adjusts_sp = false