From c404bb04f0d11511b6fa8c2c7e3e68a3fb5a3ad3 Mon Sep 17 00:00:00 2001
From: Nick Gasson <nick@nickg.me.uk>
Date: Sun, 28 Jan 2024 11:32:08 +0000
Subject: [PATCH] Optimise vcode load op in more cases

---
 src/vcode.c       | 43 ++++++++++++++++++++++++++++++-------------
 test/test_lower.c |  2 --
 2 files changed, 30 insertions(+), 15 deletions(-)

diff --git a/src/vcode.c b/src/vcode.c
index 03d7fa17..1f456d2c 100644
--- a/src/vcode.c
+++ b/src/vcode.c
@@ -3631,29 +3631,46 @@ vcode_reg_t emit_load(vcode_var_t var)
 {
    // Try scanning backwards through the block for another load or store to
    // this variable
+   enum { EAGER, CONSERVATIVE, UNSAFE } state = EAGER;
    vcode_reg_t fold = VCODE_INVALID_REG;
-   bool aliased = false;
    VCODE_FOR_EACH_OP(other) {
-      if (fold == VCODE_INVALID_REG) {
+      switch (state) {
+      case EAGER:
          if (other->kind == VCODE_OP_LOAD && other->address == var)
-            fold = other->result;
+            return other->result;
          else if (other->kind == VCODE_OP_STORE && other->address == var)
+            return other->args.items[0];
+         else if (other->kind == VCODE_OP_FCALL
+                  || other->kind == VCODE_OP_PCALL
+                  || other->kind == VCODE_OP_FILE_READ
+                  || other->kind == VCODE_OP_STORE_INDIRECT
+                  || other->kind == VCODE_OP_DEALLOCATE)
+            state = CONSERVATIVE;   // May write to variable
+         break;
+
+      case CONSERVATIVE:
+         if (other->kind == VCODE_OP_LOAD && other->address == var
+             && fold == VCODE_INVALID_REG)
+            fold = other->result;
+         else if (other->kind == VCODE_OP_STORE && other->address == var
+                  && fold == VCODE_INVALID_REG)
             fold = other->args.items[0];
-      }
+         else if (other->kind == VCODE_OP_INDEX && other->address == var)
+            state = UNSAFE;
+         else if (other->kind == VCODE_OP_CONTEXT_UPREF && other->hops == 0)
+            state = UNSAFE;   // Nested call captures variables
+         break;
 
-      if (other->kind == VCODE_OP_INDEX && other->address == var)
-         aliased = true;
-      else if (other->kind == VCODE_OP_FCALL || other->kind == VCODE_OP_PCALL)
-         break;   // Nested call captures variables
-      else if (other->kind == VCODE_OP_FILE_READ)
-         break;   // May write to variable
+      case UNSAFE:
+         break;
+      }
    }
 
-   var_t *v = vcode_var_data(var);
-
-   if (fold != VCODE_INVALID_REG && !aliased)
+   if (fold != VCODE_INVALID_REG && state != UNSAFE)
       return fold;
 
+   var_t *v = vcode_var_data(var);
+
    op_t *op = vcode_add_op(VCODE_OP_LOAD);
    op->address = var;
    op->result  = vcode_add_reg(v->type);
diff --git a/test/test_lower.c b/test/test_lower.c
index ab2a5e40..133d4d42 100644
--- a/test/test_lower.c
+++ b/test/test_lower.c
@@ -2974,7 +2974,6 @@ START_TEST(test_hintbug)
       { VCODE_OP_CONST, .value = 1 },
       { VCODE_OP_CONST, .value = 0 },
       { VCODE_OP_WRAP },
-      { VCODE_OP_LOAD, .name = "X" },
       { VCODE_OP_CONST, .value = 0 },
       { VCODE_OP_CONST, .value = 1 },
       { VCODE_OP_ALLOC },
@@ -3015,7 +3014,6 @@ START_TEST(test_issue351)
       { VCODE_OP_FCALL, .func = "*WORK.ISSUE351.DUMP_WORDS" },
       { VCODE_OP_LOAD, .name = "*right" },
       { VCODE_OP_LOAD, .name = "*step" },
-      { VCODE_OP_LOAD, .name = "I.LOOP1" },
       { VCODE_OP_ADD },
       { VCODE_OP_STORE, .name = "I.LOOP1" },
       { VCODE_OP_CMP, .cmp = VCODE_CMP_EQ },
-- 
2.39.2