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[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
This commit is contained in:
Martin Storsjö 2021-09-16 13:31:31 +03:00
parent 338f21a4bd
commit d13d9da1fb
2 changed files with 7 additions and 2 deletions
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6
clang/lib/Driver/ToolChains/Arch/ARM.cpp
View File

@ -779,7 +779,8 @@ fp16_fml_fallthrough:
// 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 @@ fp16_fml_fallthrough:
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

3
clang/test/Driver/arm-alignment.c
View File

@ -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