[clang] d13d9da - [clang] [ARM] Don't set the strict alignment flag for armv7 on Windows

[Martin Storsjö via cfe-commits]

Author: Martin Storsjö
Date: 2021-09-17T21:39:25+03:00
New Revision: d13d9da1fbe1a750f9c4fc3f4da31c9d16a530d3

URL: https://github.com/llvm/llvm-project/commit/d13d9da1fbe1a750f9c4fc3f4da31c9d16a530d3
DIFF: https://github.com/llvm/llvm-project/commit/d13d9da1fbe1a750f9c4fc3f4da31c9d16a530d3.diff

LOG: [clang] [ARM] Don't set the strict alignment flag for armv7 on Windows

Windows on armv7 is as alignment tolerant as Linux.

The alignment considerations in the Windows on ARM ABI are documented
at https://docs.microsoft.com/en-us/cpp/build/overview-of-arm-abi-conventions?view=msvc-160#alignment.

The document doesn't explicitly say in which state the OS configures
the SCTLR.A register (and it's not accessible from user space to
inspect), but in practice, unaligned loads/stores do work and seem
to be as fast as aligned loads and stores. (Unaligned strd also does
seem to work, contrary to Linux, but significantly slower, as they're
handled by the kernel - exactly as the document describes.)

Differential Revision: https://reviews.llvm.org/D109960

Added: 
    

Modified: 
    clang/lib/Driver/ToolChains/Arch/ARM.cpp
    clang/test/Driver/arm-alignment.c

Removed: 
    


################################################################################
diff  --git a/clang/lib/Driver/ToolChains/Arch/ARM.cpp b/clang/lib/Driver/ToolChains/Arch/ARM.cpp
index a64fc30858748..05d83c91ba0ef 100644
--- a/clang/lib/Driver/ToolChains/Arch/ARM.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/ARM.cpp
@@ -779,7 +779,8 @@ void arm::getARMTargetFeatures(const Driver &D, const llvm::Triple &Triple,
     // which raises an alignment fault on unaligned accesses. Linux
     // 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.
+    // Linux targets support unaligned accesses. The same goes for NaCl
+    // and Windows.
     //
     // The above behavior is consistent with GCC.
     int VersionNum = getARMSubArchVersionNumber(Triple);
@@ -787,7 +788,8 @@ void arm::getARMTargetFeatures(const Driver &D, const llvm::Triple &Triple,
       if (VersionNum < 6 ||
           Triple.getSubArch() == llvm::Triple::SubArchType::ARMSubArch_v6m)
         Features.push_back("+strict-align");
-    } else if (Triple.isOSLinux() || Triple.isOSNaCl()) {
+    } else if (Triple.isOSLinux() || Triple.isOSNaCl() ||
+               Triple.isOSWindows()) {
       if (VersionNum < 7)
         Features.push_back("+strict-align");
     } else

diff  --git a/clang/test/Driver/arm-alignment.c b/clang/test/Driver/arm-alignment.c
index b2bc8a35dfc6f..41dbb35b1c2c2 100644
--- a/clang/test/Driver/arm-alignment.c
+++ b/clang/test/Driver/arm-alignment.c
@@ -19,6 +19,9 @@
 // RUN: %clang -target armv7-unknown-nacl-gnueabihf -### %s 2> %t
 // RUN: FileCheck --check-prefix=CHECK-UNALIGNED-ARM < %t %s
 
+// RUN: %clang -target armv7-windows -### %s 2> %t
+// RUN: FileCheck --check-prefix=CHECK-UNALIGNED-ARM < %t %s
+
 // RUN: %clang -target aarch64-none-gnueabi -munaligned-access -### %s 2> %t
 // RUN: FileCheck --check-prefix=CHECK-UNALIGNED-AARCH64 < %t %s