From df230dc830a457ffa52b86f7637a6c9b225266e7 Mon Sep 17 00:00:00 2001
From: Pekka Enberg <penberg@iki.fi>
Date: Mon, 24 Jun 2024 21:43:19 +0300
Subject: [PATCH] Fix DecrJumpZero usage in translate_select()

SQLite special-cases `LIMIT 0` by emitting an explicit `Goto` to avoid
executing any `ResultRow` statements. We, however, hacked around this in
DecrJumpZero but also the placement of the generated instruction.

Let's follow SQLite codegen here. We still need to fix `LIMIT 0`,
though.
---
 core/translate.rs |  2 +-
 core/vdbe.rs      | 11 +++++------
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/core/translate.rs b/core/translate.rs
index 6d76f131d..54a4f88e8 100644
--- a/core/translate.rs
+++ b/core/translate.rs
@@ -97,13 +97,13 @@ fn translate_select(schema: &Schema, select: Select) -> Result<Program> {
             program.emit_insn(Insn::OpenReadAwait);
             program.emit_insn(Insn::RewindAsync { cursor_id });
             let rewind_await_offset = program.emit_placeholder();
-            let limit_decr_insn = limit_reg.map(|_| program.emit_placeholder());
             let (register_start, register_end) =
                 translate_columns(&mut program, Some(cursor_id), Some(table), columns);
             program.emit_insn(Insn::ResultRow {
                 register_start,
                 register_end,
             });
+            let limit_decr_insn = limit_reg.map(|_| program.emit_placeholder());
             program.emit_insn(Insn::NextAsync { cursor_id });
             program.emit_insn(Insn::NextAwait {
                 cursor_id,
diff --git a/core/vdbe.rs b/core/vdbe.rs
index 6b39a5ed1..26c69ca93 100644
--- a/core/vdbe.rs
+++ b/core/vdbe.rs
@@ -94,8 +94,6 @@ pub enum Insn {
     },
 
     // Decrement the given register and jump to the given PC if the result is zero.
-    //
-    // Unlike in SQLite, if register is already zero, we don't decrement, but take the jump.
     DecrJumpZero {
         reg: usize,
         target_pc: BranchOffset,
@@ -326,11 +324,12 @@ impl Program {
                 }
                 Insn::DecrJumpZero { reg, target_pc } => match state.registers[*reg] {
                     OwnedValue::Integer(n) => {
-                        if n > 0 {
-                            state.registers[*reg] = OwnedValue::Integer(n - 1);
-                            state.pc += 1;
-                        } else {
+                        let n = n - 1;
+                        if n == 0 {
                             state.pc = *target_pc;
+                        } else {
+                            state.registers[*reg] = OwnedValue::Integer(n);
+                            state.pc += 1;
                         }
                     }
                     _ => unreachable!("DecrJumpZero on non-integer register"),