Emit diagnostic for -munaligned-access on v6m targets

[bfoster]

Summary

Rather than silently disabling unaligned accesses for v6m targets as in the previous patch to llvm, instead produce a warning saying that this architecture doesn't support unaligned accesses.

[bfoster]

As suggested by James, we catch this case in clang rather than silently disabling unaligned accesses in the backend.

[bfoster]

Pinging for review.

[t.p.northover]

Hi,

This seems fine to me. Sorry about the delay.

Tim.

[olista01]

Committed revision 215531.

lib/Target/ARM/ARMSubtarget.cpp

case DefaultAlign:
       // 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() ||
                           isTargetNetBSD())) ||
           (hasV6Ops() && (isTargetMachO() || isTargetNetBSD()));
-      // The one exception is cortex-m0, which despite being v6, does not
-      // support unaligned accesses. Rather than make the above boolean
-      // expression even more obtuse, just override the value here.
-      if (isThumb1Only() && isMClass())
-        AllowsUnalignedMem = false;
       break;
     case StrictAlign:
       AllowsUnalignedMem = false;
       break;
     case NoStrictAlign:
       AllowsUnalignedMem = true;
       break;
   }
+  // The exception is v6m, which does not support unaligned accesses. Rather
+  // than make the above boolean expression even more obtuse, just override the
+  // value here.
+  if (isThumb1Only() && isMClass())
+    AllowsUnalignedMem = false;
 
   switch (IT) {
   case DefaultIT:
     RestrictIT = hasV8Ops() ? true : false;
     break;
   case RestrictedIT:
     RestrictIT = true;
     break;

test/CodeGen/Thumb/cortex-m0-unaligned-access.ll

-; RUN: llc -mtriple=thumbv6m-apple-unknown-macho < %s | FileCheck --check-prefix=V6M %s
-; RUN: llc -mtriple=thumbv7m-apple-unknown-macho < %s | FileCheck --check-prefix=V7M %s
-
-define i32 @split_load(i32* %p) nounwind {
-; V6M-LABEL: split_load
-; V6M: ldrh
-; V6M: ldrh
-; V7M-LABEL: split_load
-; V7M-NOT: ldrh
-; V7M: bx lr
-  %val = load i32* %p, align 2
-  ret i32 %val
-}

test/CodeGen/Thumb/v6m-unaligned-access.ll

+; RUN: llc < %s -O3 -verify-machineinstrs -mtriple=thumbv6m-none-linux-gnueabi -mcpu=cortex-m0 -arm-strict-align | FileCheck %s --check-prefix=V6M
+; RUN: llc < %s -O3 -verify-machineinstrs -mtriple=thumbv6m-none-linux-gnueabi -mcpu=cortex-m0 -arm-no-strict-align | FileCheck %s --check-prefix=V6M
+; RUN: llc < %s -O3 -verify-machineinstrs -mtriple=thumbv7m-none-linux-gnueabi -mcpu=cortex-m3 -arm-strict-align | FileCheck %s --check-prefix=V7M-ALIGNED
+; RUN: llc < %s -O3 -verify-machineinstrs -mtriple=thumbv7m-none-linux-gnueabi -mcpu=cortex-m3 -arm-no-strict-align | FileCheck %s --check-prefix=V7M
+; RUN: llc -O0 -mtriple=thumbv6m-apple-unknown-macho < %s | FileCheck --check-prefix=V6M %s
+; RUN: llc -O0 -mtriple=thumbv7m-apple-unknown-macho < %s | FileCheck --check-prefix=V7M %s
+
+; This test verifies that unaligned accesses are not generated for v6m targets
+
+define i32 @func0(i32* %p) #0 {
+entry:
+  ; V7M-ALIGNED-LABEL: entry
+  ; V7M-ALIGNED: ldrh
+  ; V7M-ALIGNED: ldrh
+
+  ; V6M-LABEL: entry
+  ; V6M: ldrh
+  ; V6M: ldrh
+
+  ; V7M-LABEL: entry
+  ; V7M-NOT: ldrh
+  ; V7M: bx lr
+
+  %0 = load i32* %p, align 2
+
+  ret i32 %0
+}