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 Linux and NaCl.

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/NaCl behave sanely).

The patch keeps the -arm-strict-align command line option, and adds
-arm-no-strict-align. They behave similarly to GCC's -mstrict-align and
-mnostrict-align.

I originally encountered this discrepancy in FastIsel tests which expect
unaligned load/store generation. Overall this should slightly improve
performance in most cases because of reduced I$ pressure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182175 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 46 insertions, 10 deletions

diff --git a/lib/Target/ARM/ARMSubtarget.cpp b/lib/Target/ARM/ARMSubtarget.cpp
index 8653c462f0..c7d9743790 100644
--- a/lib/Target/ARM/ARMSubtarget.cpp
+++ b/lib/Target/ARM/ARMSubtarget.cpp
@@ -38,9 +38,24 @@ static cl::opt<bool>
 UseFusedMulOps("arm-use-mulops",
                cl::init(true), cl::Hidden);
 
-static cl::opt<bool>
-StrictAlign("arm-strict-align", cl::Hidden,
-            cl::desc("Disallow all unaligned memory accesses"));
+enum AlignMode {
+  DefaultAlign,
+  StrictAlign,
+  NoStrictAlign
+};
+
+static cl::opt<AlignMode>
+Align(cl::desc("Load/store alignment support"),
+      cl::Hidden, cl::init(DefaultAlign),
+      cl::values(
+          clEnumValN(DefaultAlign,  "arm-default-align",
+                     "Generate unaligned accesses only on hardware/OS "
+                     "combinations that are known to support them"),
+          clEnumValN(StrictAlign,   "arm-strict-align",
+                     "Disallow all unaligned memory accesses"),
+          clEnumValN(NoStrictAlign, "arm-no-strict-align",
+                     "Allow unaligned memory accesses"),
+          clEnumValEnd));
 
 ARMSubtarget::ARMSubtarget(const std::string &TT, const std::string &CPU,
                            const std::string &FS, const TargetOptions &Options)
@@ -162,10 +177,32 @@ 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;
+  switch (Align) {
+    case DefaultAlign:
+      // 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+
+      // Linux targets support unaligned accesses. The same goes for NaCl.
+      //
+      // The above behavior is consistent with GCC.
+      AllowsUnalignedMem = (
+          (hasV7Ops() && (isTargetLinux() || isTargetNaCl())) ||
+          (hasV6Ops() && isTargetDarwin()));
+      break;
+    case StrictAlign:
+      AllowsUnalignedMem = false;
+      break;
+    case NoStrictAlign:
+      AllowsUnalignedMem = true;
+      break;
+  }
 
   // NEON f32 ops are non-IEEE 754 compliant. Darwin is ok with it by default.
   uint64_t Bits = getFeatureBits();
diff --git a/lib/Target/ARM/ARMSubtarget.h b/lib/Target/ARM/ARMSubtarget.h
index 038eb76ae1..d01316511c 100644
--- a/lib/Target/ARM/ARMSubtarget.h
+++ b/lib/Target/ARM/ARMSubtarget.h
@@ -270,9 +270,8 @@ public:
 
   bool isTargetIOS() const { return TargetTriple.getOS() == Triple::IOS; }
   bool isTargetDarwin() const { return TargetTriple.isOSDarwin(); }
-  bool isTargetNaCl() const {
-    return TargetTriple.getOS() == Triple::NaCl;
-  }
+  bool isTargetNaCl() const { return TargetTriple.getOS() == Triple::NaCl; }
+  bool isTargetLinux() const { return TargetTriple.getOS() == Triple::Linux; }
   bool isTargetELF() const { return !isTargetDarwin(); }
 
   bool isAPCS_ABI() const { return TargetABI == ARM_ABI_APCS; }