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

[Paul Kirth via cfe-commits]

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

>From 20634c2f54ae667ee8374d12e58e582aa6cdd051 Mon Sep 17 00:00:00 2001
From: Paul Kirth <paulkirth at google.com>
Date: Tue, 20 Feb 2024 10:34:57 -0800
Subject: [PATCH 1/2] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20in?=
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Created using spr 1.3.4
---
 clang/lib/Driver/ToolChains/Arch/ARM.cpp | 14 ++++++--------
 clang/test/Driver/arm-alignment.c        |  8 ++++++++
 2 files changed, 14 insertions(+), 8 deletions(-)

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
 

>From a078e658bdcd851d87331ecf87224dc2ddb13c69 Mon Sep 17 00:00:00 2001
From: Paul Kirth <paulkirth at google.com>
Date: Tue, 20 Feb 2024 10:38:50 -0800
Subject: [PATCH 2/2] git clang-format

Created using spr 1.3.4
---
 clang/lib/Driver/ToolChains/Arch/ARM.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/lib/Driver/ToolChains/Arch/ARM.cpp b/clang/lib/Driver/ToolChains/Arch/ARM.cpp
index 3bf6056f0c3ecc..a1aea397925931 100644
--- a/clang/lib/Driver/ToolChains/Arch/ARM.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/ARM.cpp
@@ -906,7 +906,7 @@ llvm::ARM::FPUKind arm::getARMTargetFeatures(const Driver &D,
           Triple.getSubArch() == llvm::Triple::SubArchType::ARMSubArch_v6m)
         Features.push_back("+strict-align");
     } else if (VersionNum < 7)
-        Features.push_back("+strict-align");
+      Features.push_back("+strict-align");
   }
 
   // llvm does not support reserving registers in general. There is support