Support unaligned load/store on more ARM targets

This patch matches GCC behavior: the code used to only allow unaligned
load/store on ARM for v6+ Darwin, it will now allow unaligned load/store for
v6+ Darwin as well as for v7+ on other targets.

The distinction is made because v6 doesn't guarantee support (but LLVM assumes
that Apple controls hardware+kernel and therefore have conformant v6 CPUs),
whereas v7 does provide this guarantee (and Linux behaves sanely).

Overall this should slightly improve performance in most cases because of
reduced I$ pressure.

Patch by JF Bastien

llvm-svn: 181897

1 files changed, 17 insertions, 4 deletions

diff --git a/llvm/lib/Target/ARM/ARMSubtarget.cpp b/llvm/lib/Target/ARM/ARMSubtarget.cpp
index 8653c462f06..9ff0d61481c 100644
--- a/llvm/lib/Target/ARM/ARMSubtarget.cpp
+++ b/llvm/lib/Target/ARM/ARMSubtarget.cpp
@@ -162,10 +162,23 @@ void ARMSubtarget::resetSubtargetFeatures(StringRef CPU, StringRef FS) {
   if (!isThumb() || hasThumb2())
     PostRAScheduler = true;
 
-  // v6+ may or may not support unaligned mem access depending on the system
-  // configuration.
-  if (!StrictAlign && hasV6Ops() && isTargetDarwin())
-    AllowsUnalignedMem = true;
+  if (!StrictAlign) {
+    // Assume pre-ARMv6 doesn't support unaligned accesses.
+    //
+    // ARMv6 may or may not support unaligned accesses depending on the
+    // SCTLR.U bit, which is architecture-specific. We assume ARMv6
+    // Darwin targets support unaligned accesses, and others don't.
+    //
+    // ARMv7 always has SCTLR.U set to 1, but it has a new SCTLR.A bit
+    // 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+
+    // targets support unaligned accesses.
+    //
+    // The above behavior is consistent with GCC.
+    if (hasV7Ops() || (hasV6Ops() && isTargetDarwin()))
+      AllowsUnalignedMem = true;
+  }
 
   // NEON f32 ops are non-IEEE 754 compliant. Darwin is ok with it by default.
   uint64_t Bits = getFeatureBits();