diff --git a/src/cfg.rs b/src/cfg.rs
index 7769556..a882f97 100644
--- a/src/cfg.rs
+++ b/src/cfg.rs
@@ -5,7 +5,7 @@
 
 //! Lightweight CFG analyses.
 
-use crate::{domtree, postorder, Block, Function, Inst, OperandKind, ProgPoint, RegAllocError};
+use crate::{domtree, postorder, Block, Function, Inst, ProgPoint, RegAllocError};
 use smallvec::{smallvec, SmallVec};
 
 #[derive(Clone, Debug)]
@@ -16,12 +16,6 @@ pub struct CFGInfo {
     pub domtree: Vec<Block>,
     /// For each instruction, the block it belongs to.
     pub insn_block: Vec<Block>,
-    /// For each vreg, the instruction that defines it, if any.
-    pub vreg_def_inst: Vec<Inst>,
-    /// For each vreg, the block that defines it as a blockparam, if
-    /// any. (Every vreg must have a valid entry in either
-    /// `vreg_def_inst` or `vreg_def_blockparam`.)
-    pub vreg_def_blockparam: Vec<(Block, u32)>,
     /// For each block, the first instruction.
     pub block_entry: Vec<ProgPoint>,
     /// For each block, the last instruction.
@@ -48,8 +42,6 @@ impl CFGInfo {
             f.entry_block(),
         );
         let mut insn_block = vec![Block::invalid(); f.num_insts()];
-        let mut vreg_def_inst = vec![Inst::invalid(); f.num_vregs()];
-        let mut vreg_def_blockparam = vec![(Block::invalid(), 0); f.num_vregs()];
         let mut block_entry = vec![ProgPoint::before(Inst::invalid()); f.num_blocks()];
         let mut block_exit = vec![ProgPoint::before(Inst::invalid()); f.num_blocks()];
         let mut backedge_in = vec![0; f.num_blocks()];
@@ -57,19 +49,8 @@ impl CFGInfo {
 
         for block in 0..f.num_blocks() {
             let block = Block::new(block);
-            for (i, param) in f.block_params(block).iter().enumerate() {
-                vreg_def_blockparam[param.vreg()] = (block, i as u32);
-            }
             for inst in f.block_insns(block).iter() {
                 insn_block[inst.index()] = block;
-                for operand in f.inst_operands(inst) {
-                    match operand.kind() {
-                        OperandKind::Def => {
-                            vreg_def_inst[operand.vreg().vreg()] = inst;
-                        }
-                        _ => {}
-                    }
-                }
             }
             block_entry[block.index()] = ProgPoint::before(f.block_insns(block).first());
             block_exit[block.index()] = ProgPoint::after(f.block_insns(block).last());
@@ -139,8 +120,6 @@ impl CFGInfo {
             postorder,
             domtree,
             insn_block,
-            vreg_def_inst,
-            vreg_def_blockparam,
             block_entry,
             block_exit,
             approx_loop_depth,
diff --git a/src/fuzzing/mod.rs b/src/fuzzing/mod.rs
index e6a225e..1ab9d6d 100644
--- a/src/fuzzing/mod.rs
+++ b/src/fuzzing/mod.rs
@@ -6,6 +6,7 @@
 //! Utilities for fuzzing.
 
 pub mod func;
+pub mod ssa;
 
 // Re-exports for fuzz targets.
 
@@ -21,9 +22,6 @@ pub mod moves {
 pub mod cfg {
     pub use crate::cfg::*;
 }
-pub mod ssa {
-    pub use crate::ssa::*;
-}
 pub mod ion {
     pub use crate::ion::*;
 }
diff --git a/src/ssa.rs b/src/fuzzing/ssa.rs
similarity index 75%
rename from src/ssa.rs
rename to src/fuzzing/ssa.rs
index 44ef681..21852dc 100644
--- a/src/ssa.rs
+++ b/src/fuzzing/ssa.rs
@@ -10,6 +10,30 @@ use crate::cfg::CFGInfo;
 use crate::{Block, Function, Inst, OperandKind, RegAllocError};
 
 pub fn validate_ssa<F: Function>(f: &F, cfginfo: &CFGInfo) -> Result<(), RegAllocError> {
+    // For each vreg, the instruction that defines it, if any.
+    let mut vreg_def_inst = vec![Inst::invalid(); f.num_vregs()];
+    // For each vreg, the block that defines it as a blockparam, if
+    // any. (Every vreg must have a valid entry in either
+    // `vreg_def_inst` or `vreg_def_blockparam`.)
+    let mut vreg_def_blockparam = vec![(Block::invalid(), 0); f.num_vregs()];
+
+    for block in 0..f.num_blocks() {
+        let block = Block::new(block);
+        for (i, param) in f.block_params(block).iter().enumerate() {
+            vreg_def_blockparam[param.vreg()] = (block, i as u32);
+        }
+        for inst in f.block_insns(block).iter() {
+            for operand in f.inst_operands(inst) {
+                match operand.kind() {
+                    OperandKind::Def => {
+                        vreg_def_inst[operand.vreg().vreg()] = inst;
+                    }
+                    _ => {}
+                }
+            }
+        }
+    }
+
     // Walk the blocks in arbitrary order. Check, for every use, that
     // the def is either in the same block in an earlier inst, or is
     // defined (by inst or blockparam) in some other block that
@@ -29,10 +53,10 @@ pub fn validate_ssa<F: Function>(f: &F, cfginfo: &CFGInfo) -> Result<(), RegAllo
             for operand in operands {
                 match operand.kind() {
                     OperandKind::Use => {
-                        let def_block = if cfginfo.vreg_def_inst[operand.vreg().vreg()].is_valid() {
-                            cfginfo.insn_block[cfginfo.vreg_def_inst[operand.vreg().vreg()].index()]
+                        let def_block = if vreg_def_inst[operand.vreg().vreg()].is_valid() {
+                            cfginfo.insn_block[vreg_def_inst[operand.vreg().vreg()].index()]
                         } else {
-                            cfginfo.vreg_def_blockparam[operand.vreg().vreg()].0
+                            vreg_def_blockparam[operand.vreg().vreg()].0
                         };
                         if def_block.is_invalid() {
                             return Err(RegAllocError::SSA(operand.vreg(), iix));
diff --git a/src/lib.rs b/src/lib.rs
index 2243f08..113eca9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -34,7 +34,6 @@ pub mod indexset;
 pub(crate) mod ion;
 pub(crate) mod moves;
 pub(crate) mod postorder;
-pub(crate) mod ssa;
 
 #[macro_use]
 mod index;