Place .xip sections into flash

We added the `.xip` section to ensure that the reset handler and
pre-init functions would be placed in flash. This commit lets users
place other content into that section.

`.xip` is intended for instructions. The runtime builder will place
these instructions into the same load region as `.text`. However,
there's no pre-`main` relocation.

Aligning the `.xip` and the `.text` section produces more predictable
behavior between GNU's ld and LLVM's lld.

4 files changed, 46 insertions, 11 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index bd6fb98..2920663 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 ## [Unreleased]
 
+Place sections starting with `.xip` into the same load region of `.text`.
+Unlike `.text`, the contents in `.xip` will not be relocated.
+
+Four byte align both the `.xip` and `.text` sections for more predictable
+behaviors across linkers.
+
 ## [0.1.6] 2025-03-01
 
 Add new MCU targets:
diff --git a/examples/blink-rtic.rs b/examples/blink-rtic.rs
index 04446e3..e463f33 100644
--- a/examples/blink-rtic.rs
+++ b/examples/blink-rtic.rs
@@ -12,6 +12,13 @@ use imxrt_rt as _;
 /// This is checked in an automated test.
 static mut DATA: u32 = 5;
 
+#[unsafe(link_section = ".xip")]
+#[unsafe(no_mangle)]
+#[inline(never)]
+fn increment_data() {
+    unsafe { crate::DATA = crate::DATA.wrapping_add(1) };
+}
+
 #[rtic::app(device = board::rtic_support, peripherals = false)]
 mod app {
     const PIT_PERIOD_US: u32 = 1_000_000;
@@ -35,7 +42,7 @@ mod app {
 
     #[task(binds = PIT, local = [led, pit])]
     fn pit(cx: pit::Context) {
-        unsafe { crate::DATA += 1 };
+        crate::increment_data();
         cx.local.led.toggle();
         cx.local.pit.clear_interrupts();
     }
diff --git a/src/host/imxrt-link.x b/src/host/imxrt-link.x
index a389325..dfe7355 100644
--- a/src/host/imxrt-link.x
+++ b/src/host/imxrt-link.x
@@ -70,14 +70,15 @@ SECTIONS
   __sivector_table = LOADADDR(.vector_table);
 
   /* This section guarantees VMA = LMA to allow the execute-in-place entry point to be inside the image. */
-  .xip :
+  .xip : ALIGN(4)
   {
     /* Included here if not otherwise included in the boot header. */
     *(.Reset);
     *(.__pre_init);
+    *(.xip .xip.*);
   } > REGION_LOAD_TEXT
 
-  .text :
+  .text : ALIGN(4)
   {
     FILL(0xff);
     __stext = .;
diff --git a/tests/inspect_elf.rs b/tests/inspect_elf.rs
index cdb3245..e9211eb 100644
--- a/tests/inspect_elf.rs
+++ b/tests/inspect_elf.rs
@@ -239,18 +239,19 @@ fn imxrt1010evk() {
     );
     assert_eq!(binary.section_lma(".vector_table"), 0x6000_2000);
 
+    let xip = binary.section(".xip").unwrap();
     let text = binary.section(".text").unwrap();
     assert_eq!(text.address, ITCM, "text");
     assert_eq!(
         binary.section_lma(".text"),
-        0x6000_2000 + vector_table.size,
+        aligned(0x6000_2000 + vector_table.size + xip.size, 4),
         "text VMA expected behind vector table"
     );
 
     let rodata = binary.section(".rodata").unwrap();
     assert_eq!(
         rodata.address,
-        0x6000_2000 + vector_table.size + aligned(text.size, 4),
+        aligned(0x6000_2000 + vector_table.size + text.size + xip.size, 4),
         "rodata LMA & VMA expected behind text"
     );
     assert_eq!(rodata.address, binary.section_lma(".rodata"));
@@ -298,6 +299,12 @@ fn imxrt1010evk() {
     );
     assert_eq!(heap.size, 1024);
     assert_eq!(binary.section_lma(".heap"), heap.address, "Heap is NOLOAD");
+
+    let increment_data_xip = binary.symbol_value("increment_data").unwrap();
+    assert!(
+        0x6000_0000 < increment_data_xip && increment_data_xip < 0x7000_0000,
+        "increment_data is not XiP"
+    );
 }
 
 fn baseline_teensy4(binary: &ImxrtBinary, dcd_at_runtime: u32, stack_size: u64, heap_size: u64) {
@@ -334,6 +341,7 @@ fn baseline_teensy4(binary: &ImxrtBinary, dcd_at_runtime: u32, stack_size: u64,
     assert_eq!(binary.section_lma(".stack"), stack.address);
 
     let vector_table = binary.section(".vector_table").unwrap();
+    let xip = binary.section(".xip").unwrap();
     assert_eq!(
         Section {
             address: stack.address + stack.size,
@@ -349,14 +357,14 @@ fn baseline_teensy4(binary: &ImxrtBinary, dcd_at_runtime: u32, stack_size: u64,
     assert_eq!(binary.section_lma(".vector_table"), 0x6000_2000);
 
     let text = binary.section(".text").unwrap();
-    assert_eq!(
-        text.address,
-        binary.section_lma(".vector_table") + vector_table.size,
-        "text"
+    let expected_text_address = aligned(
+        binary.section_lma(".vector_table") + vector_table.size + xip.size,
+        4,
     );
+    assert_eq!(text.address, expected_text_address, "text");
     assert_eq!(
         binary.section_lma(".text"),
-        0x6000_2000 + vector_table.size,
+        aligned(0x6000_2000 + vector_table.size + xip.size, 4),
         "text VMA expected behind vector table"
     );
 
@@ -414,6 +422,12 @@ fn baseline_teensy4(binary: &ImxrtBinary, dcd_at_runtime: u32, stack_size: u64,
         "{heap_size} byte heap in DTCM behind uninit"
     );
     assert_eq!(binary.section_lma(".heap"), heap.address, "Heap is NOLOAD");
+
+    let increment_data_xip = binary.symbol_value("increment_data").unwrap();
+    assert!(
+        0x6000_0000 < increment_data_xip && increment_data_xip < 0x7000_0000,
+        "increment_data is not XiP"
+    );
 }
 
 #[test]
@@ -562,11 +576,12 @@ fn imxrt1170evk_cm7() {
     );
     assert_eq!(binary.section_lma(".vector_table"), 0x3000_2000);
 
+    let xip = binary.section(".xip").unwrap();
     let text = binary.section(".text").unwrap();
     assert_eq!(text.address, ITCM, "text");
     assert_eq!(
         binary.section_lma(".text"),
-        0x3000_2000 + vector_table.size,
+        aligned(0x3000_2000 + vector_table.size + xip.size, 4),
         "text VMA expected behind vector table"
     );
 
@@ -622,4 +637,10 @@ fn imxrt1170evk_cm7() {
         "0 byte heap in DTCM behind rodata table"
     );
     assert_eq!(binary.section_lma(".heap"), heap.address, "Heap is NOLOAD");
+
+    let increment_data_xip = binary.symbol_value("increment_data").unwrap();
+    assert!(
+        0x3000_0000 < increment_data_xip && increment_data_xip < 0x4000_0000,
+        "increment_data is not XiP"
+    );
 }