[llvm] [BOLT] Avoid unnecessary AArch64 ADR relaxation (PR #215415)

[Adam Bzowski via llvm-commits]

https://github.com/adam-bzowski-arm created https://github.com/llvm/llvm-project/pull/215415

This is a temporary fix for an AArch64 ADR/LDR relaxation issue. The correct treatment likely requires merging AArch64RelaxationPass with LongJmpPass, so address materialization and branch/stub insertion can use one final layout model.

The relaxation pass currently tries to relax every ADR/LDR in functions at least 1 MiB, including same-fragment references in non-simple functions. Since BOLT cannot grow non-simple functions without an adjacent NOP, this can fail even though the preserved layout leaves the original displacement valid.

Skip relaxation for same-fragment references in non-simple functions, while retaining the size check for reorderable simple functions.

>From f4dc6a1ee0f68f80ef39a69e5b6a8dfca59ec400 Mon Sep 17 00:00:00 2001
From: Adam Bzowski <Adam.Bzowski at arm.com>
Date: Fri, 7 Aug 2026 11:34:59 +0000
Subject: [PATCH] [BOLT] Avoid unnecessary AArch64 ADR relaxation

This is a temporary fix for an AArch64 ADR/LDR relaxation issue.
The correct treatment likely requires merging AArch64RelaxationPass with
LongJmpPass, so address materialization and branch/stub insertion can
use one final layout model.

The relaxation pass currently tries to relax every ADR/LDR in functions
at least 1 MiB, including same-fragment references in non-simple
functions. Since BOLT cannot grow non-simple functions without an
adjacent NOP, this can fail even though the preserved layout leaves the
original displacement valid.

Skip relaxation for same-fragment references in non-simple functions,
while retaining the size check for reorderable simple functions.

Change-Id: Idf44a1b80b15175a8c5bf719b6c7d996d4785bbe
---
 bolt/lib/Passes/AArch64RelaxationPass.cpp     |  8 +++--
 .../AArch64/adr-relaxation-large-non-simple.s | 30 +++++++++++++++++++
 2 files changed, 35 insertions(+), 3 deletions(-)
 create mode 100644 bolt/test/AArch64/adr-relaxation-large-non-simple.s

diff --git a/bolt/lib/Passes/AArch64RelaxationPass.cpp b/bolt/lib/Passes/AArch64RelaxationPass.cpp
index 2b7384dc848dd..51260ceadf918 100644
--- a/bolt/lib/Passes/AArch64RelaxationPass.cpp
+++ b/bolt/lib/Passes/AArch64RelaxationPass.cpp
@@ -59,10 +59,12 @@ void AArch64RelaxationPass::runOnFunction(BinaryFunction &BF) {
           continue;
       }
 
-      // Don't relax ADR/LDR if it points to the same function and is in the
-      // main fragment and BF initial size is < 1MB.
+      // The layout of a non-simple function is preserved, so references within
+      // the same fragment retain their original in-range displacement. For
+      // simple functions, basic blocks can move, but an initial size below 1MiB
+      // guarantees that internal references remain in range after reordering.
       const unsigned OneMB = 0x100000;
-      if (BF.getSize() < OneMB) {
+      if (!BF.isSimple() || BF.getSize() < OneMB) {
         BinaryFunction *TargetBF = BC.getFunctionForSymbol(Symbol);
         if (TargetBF == &BF && !BB.isSplit())
           continue;
diff --git a/bolt/test/AArch64/adr-relaxation-large-non-simple.s b/bolt/test/AArch64/adr-relaxation-large-non-simple.s
new file mode 100644
index 0000000000000..babcdb5304b98
--- /dev/null
+++ b/bolt/test/AArch64/adr-relaxation-large-non-simple.s
@@ -0,0 +1,30 @@
+## Check that an ADR targeting the same fragment is not relaxed in a large
+## non-simple function. BOLT preserves the layout of non-simple functions, so
+## the ADR displacement cannot change even when the function is larger than the
+## instruction's 1MiB range.
+
+# RUN: llvm-mc -filetype=obj -triple aarch64-unknown-unknown %s -o %t.o
+# RUN: %clang %cflags %t.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.bolt --lite=false
+
+  .text
+  .globl _start
+  .type _start, %function
+_start:
+  .cfi_startproc
+.Ladr:
+  adr x1, .Ladr
+  br x0
+
+  // Make the function's code larger than 1MiB. The unknown indirect branch
+  // makes the function non-simple, while the self-referential ADR remains in
+  // range.
+  .rept 0x40000
+  nop
+  .endr
+  ret
+  .cfi_endproc
+  .size _start, .-_start
+
+  // Force BOLT's relocation mode.
+  .reloc 0, R_AARCH64_NONE