From a8c8834a3cc4367dba5bb16c9fe7addbbfa18589 Mon Sep 17 00:00:00 2001
From: Folkert <folkert@folkertdev.nl>
Date: Sat, 16 Sep 2023 15:57:03 +0200
Subject: [PATCH] actually understand this jmp address bug

---
 .../compiler/gen_dev/src/generic64/aarch64.rs | 26 +++----------------
 crates/compiler/gen_dev/src/generic64/mod.rs  | 14 +++++++++-
 .../compiler/gen_dev/src/generic64/x86_64.rs  | 23 +++-------------
 3 files changed, 20 insertions(+), 43 deletions(-)

diff --git a/crates/compiler/gen_dev/src/generic64/aarch64.rs b/crates/compiler/gen_dev/src/generic64/aarch64.rs
index 010a0c9d18..7d7c955f4f 100644
--- a/crates/compiler/gen_dev/src/generic64/aarch64.rs
+++ b/crates/compiler/gen_dev/src/generic64/aarch64.rs
@@ -1116,27 +1116,8 @@ impl Assembler<AArch64GeneralReg, AArch64FloatReg> for AArch64Assembler {
             todo!("jump offsets over 27 bits for AArch64: {:#x}", offset);
         }
 
-        buf.len()
-    }
-
-    /// Updates a jump instruction to a new offset and returns the number of bytes written.
-    fn update_jmp_imm32_offset(
-        buf: &mut Vec<'_, u8>,
-        jmp_location: u64,
-        base_offset: u64,
-        target_offset: u64,
-    ) {
-        let old_buf_len = buf.len();
-
-        // write the jmp at the back of buf
-        let jmp_offset = target_offset as i32 - base_offset as i32 + 4;
-        Self::jmp_imm32(buf, jmp_offset);
-
-        // move the new jmp instruction into position
-        buf.copy_within(old_buf_len.., jmp_location as usize);
-
-        // wipe the jmp we created at the end
-        buf.truncate(old_buf_len)
+        // on aarch64, jumps are calculated from the start of the jmp instruction
+        buf.len() - 4
     }
 
     #[inline(always)]
@@ -1172,7 +1153,8 @@ impl Assembler<AArch64GeneralReg, AArch64FloatReg> for AArch64Assembler {
             todo!("jump offsets over 20 bits for AArch64: {:#x}", offset);
         }
 
-        buf.len()
+        // on aarch64, jumps are calculated from the start of the jmp instruction
+        buf.len() - 4
     }
 
     #[inline(always)]
diff --git a/crates/compiler/gen_dev/src/generic64/mod.rs b/crates/compiler/gen_dev/src/generic64/mod.rs
index 026fa7f5e1..131a3c6704 100644
--- a/crates/compiler/gen_dev/src/generic64/mod.rs
+++ b/crates/compiler/gen_dev/src/generic64/mod.rs
@@ -263,7 +263,19 @@ pub trait Assembler<GeneralReg: RegTrait, FloatReg: RegTrait>: Sized + Copy {
         jmp_location: u64,
         base_offset: u64,
         target_offset: u64,
-    );
+    ) {
+        let old_buf_len = buf.len();
+
+        // write the jmp at the back of buf
+        let jmp_offset = target_offset as i32 - base_offset as i32;
+        Self::jmp_imm32(buf, jmp_offset);
+
+        // move the new jmp instruction into position
+        buf.copy_within(old_buf_len.., jmp_location as usize);
+
+        // wipe the jmp we created at the end
+        buf.truncate(old_buf_len)
+    }
 
     fn tail_call(buf: &mut Vec<'_, u8>) -> u64;
 
diff --git a/crates/compiler/gen_dev/src/generic64/x86_64.rs b/crates/compiler/gen_dev/src/generic64/x86_64.rs
index dcfb26c904..793849241a 100644
--- a/crates/compiler/gen_dev/src/generic64/x86_64.rs
+++ b/crates/compiler/gen_dev/src/generic64/x86_64.rs
@@ -2111,29 +2111,11 @@ impl Assembler<X86_64GeneralReg, X86_64FloatReg> for X86_64Assembler {
     #[inline(always)]
     fn jmp_imm32(buf: &mut Vec<'_, u8>, offset: i32) -> usize {
         jmp_imm32(buf, offset);
+
+        // on x86_64, jumps are calculated from the end of the jmp instruction
         buf.len()
     }
 
-    /// Updates a jump instruction to a new offset and returns the number of bytes written.
-    fn update_jmp_imm32_offset(
-        buf: &mut Vec<'_, u8>,
-        jmp_location: u64,
-        base_offset: u64,
-        target_offset: u64,
-    ) {
-        let old_buf_len = buf.len();
-
-        // write the jmp at the back of buf
-        let jmp_offset = target_offset as i32 - base_offset as i32;
-        Self::jmp_imm32(buf, jmp_offset);
-
-        // move the new jmp instruction into position
-        buf.copy_within(old_buf_len.., jmp_location as usize);
-
-        // wipe the jmp we created at the end
-        buf.truncate(old_buf_len)
-    }
-
     #[inline(always)]
     fn tail_call(buf: &mut Vec<'_, u8>) -> u64 {
         Self::jmp_imm32(buf, 0);
@@ -2164,6 +2146,7 @@ impl Assembler<X86_64GeneralReg, X86_64FloatReg> for X86_64Assembler {
 
         jne_imm32(buf, offset);
 
+        // on x86_64, jumps are calculated from the end of the jmp instruction
         buf.len()
     }