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
@@ -513,6 +514,7 @@ impl<I: VCodeInst> VCode<I> {
             buffer.bind_label(MachLabel::from_block(block));
             label_insn_iix[block as usize] = start;
 
+            if cfg_metadata {
                 // Track BB starts. If we have backed up due to MachBuffer
                 // branch opts, note that the removed blocks were removed.
                 let cur_offset = buffer.cur_offset();
@@ -526,6 +528,7 @@ impl<I: VCodeInst> VCode<I> {
                 }
                 bb_starts.push(Some(cur_offset));
                 last_offset = Some(cur_offset);
+            }
 
             for iix in start..end {
                 let srcloc = self.srclocs[iix as usize];
@@ -606,6 +609,7 @@ impl<I: VCodeInst> VCode<I> {
         // Create `bb_edges` and final (filtered) `bb_starts`.
         let mut final_bb_starts = vec![];
         let mut bb_edges = vec![];
+        if cfg_metadata {
             for block in 0..self.num_blocks() {
                 if bb_starts[block].is_none() {
                     // Block was deleted by MachBuffer; skip.
@@ -621,6 +625,7 @@ impl<I: VCodeInst> VCode<I> {
                     bb_edges.push((from, to));
                 }
             }
+        }
 
         (buffer, final_bb_starts, bb_edges)
     }

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