[clang] [clang][driver] Allow unaligned access on ARMv7 and higher by default (PR #82400)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-arm

Author: Paul Kirth (ilovepi)

<details>
<summary>Changes</summary>

ARM's Clang and GCC embedded compilers default to allowing unaligned
access for ARMv7+. This patch changes the Clang driver default to match.
Users can opt out with `-mno-unaligned-access`.

Fixes #<!-- -->59560


---
Full diff: https://github.com/llvm/llvm-project/pull/82400.diff


2 Files Affected:

- (modified) clang/lib/Driver/ToolChains/Arch/ARM.cpp (+6-8) 
- (modified) clang/test/Driver/arm-alignment.c (+8) 


``````````diff
diff --git a/clang/lib/Driver/ToolChains/Arch/ARM.cpp b/clang/lib/Driver/ToolChains/Arch/ARM.cpp
index e6ee2f88a84edf..3bf6056f0c3ecc 100644
--- a/clang/lib/Driver/ToolChains/Arch/ARM.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/ARM.cpp
@@ -895,20 +895,18 @@ llvm::ARM::FPUKind arm::getARMTargetFeatures(const Driver &D,
     // defaults this bit to 0 and handles it as a system-wide (not
     // per-process) setting. It is therefore safe to assume that ARMv7+
     // Linux targets support unaligned accesses. The same goes for NaCl
-    // and Windows.
-    //
-    // The above behavior is consistent with GCC.
+    // and Windows. However, ARM's forks of GCC and Clang both allow
+    // unaligned accesses by default for all targets. We follow this
+    // behavior and enable unaligned accesses by default for ARMv7+ targets.
+    // Users can disable behavior via compiler options (-mno-unaliged-access).
+    // See https://github.com/llvm/llvm-project/issues/59560 for more info.
     int VersionNum = getARMSubArchVersionNumber(Triple);
     if (Triple.isOSDarwin() || Triple.isOSNetBSD()) {
       if (VersionNum < 6 ||
           Triple.getSubArch() == llvm::Triple::SubArchType::ARMSubArch_v6m)
         Features.push_back("+strict-align");
-    } else if (Triple.isOSLinux() || Triple.isOSNaCl() ||
-               Triple.isOSWindows()) {
-      if (VersionNum < 7)
+    } else if (VersionNum < 7)
         Features.push_back("+strict-align");
-    } else
-      Features.push_back("+strict-align");
   }
 
   // llvm does not support reserving registers in general. There is support
diff --git a/clang/test/Driver/arm-alignment.c b/clang/test/Driver/arm-alignment.c
index 9177b625729b85..6d0084451e82c7 100644
--- a/clang/test/Driver/arm-alignment.c
+++ b/clang/test/Driver/arm-alignment.c
@@ -22,6 +22,14 @@
 // RUN: %clang -target armv7-windows -### %s 2> %t
 // RUN: FileCheck --check-prefix=CHECK-UNALIGNED-ARM < %t %s
 
+/// Ensure that by default before ARMv7 we default to +strict-align
+// RUN: %clang -target armv6 -### %s 2> %t
+// RUN: FileCheck --check-prefix=CHECK-ALIGNED-ARM < %t %s
+
+/// After ARMv7 by default we allow unaligned accesses for all targets
+// RUN: %clang -target armv7 -### %s 2> %t
+// RUN: FileCheck --check-prefix=CHECK-UNALIGNED-ARM < %t %s
+
 // RUN: %clang --target=aarch64 -munaligned-access -### %s 2> %t
 // RUN: FileCheck --check-prefix=CHECK-UNALIGNED-AARCH64 < %t %s
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/82400