From: Tim Northover <tnorthover@apple.com>
Date: Wed, 3 Jul 2013 09:20:36 +0000 (+0000)
Subject: ARM: relax the atomic release barrier to "dmb ishst" on Swift
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=a10c01a6c62792be825c562314a646437b21bfec;p=oota-llvm.git

ARM: relax the atomic release barrier to "dmb ishst" on Swift

Swift cores implement store barriers that are stronger than the ARM
specification but weaker than general barriers. They are, in fact, just about
enough to provide the ordering needed for atomic operations with release
semantics.

This patch makes use of that quirk.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185527 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Target/ARM/ARMISelLowering.cpp b/lib/Target/ARM/ARMISelLowering.cpp
index ff8571ba033..cc09754ef70 100644
--- a/lib/Target/ARM/ARMISelLowering.cpp
+++ b/lib/Target/ARM/ARMISelLowering.cpp
@@ -2557,8 +2557,18 @@ static SDValue LowerATOMIC_FENCE(SDValue Op, SelectionDAG &DAG,
                        DAG.getConstant(0, MVT::i32));
   }
 
+  ConstantSDNode *OrdN = cast<ConstantSDNode>(Op.getOperand(1));
+  AtomicOrdering Ord = static_cast<AtomicOrdering>(OrdN->getZExtValue());
+  unsigned Domain = ARM_MB::ISH;
+  if (Subtarget->isSwift() && Ord == Release) {
+    // Swift happens to implement ISHST barriers in a way that's compatible with
+    // Release semantics but weaker than ISH so we'd be fools not to use
+    // it. Beware: other processors probably don't!
+    Domain = ARM_MB::ISHST;
+  }
+
   return DAG.getNode(ARMISD::MEMBARRIER, dl, MVT::Other, Op.getOperand(0),
-                     DAG.getConstant(ARM_MB::ISH, MVT::i32));
+                     DAG.getConstant(Domain, MVT::i32));
 }
 
 static SDValue LowerPREFETCH(SDValue Op, SelectionDAG &DAG,
diff --git a/test/CodeGen/ARM/atomic-64bit.ll b/test/CodeGen/ARM/atomic-64bit.ll
index f2c7305ff33..669c56302fb 100644
--- a/test/CodeGen/ARM/atomic-64bit.ll
+++ b/test/CodeGen/ARM/atomic-64bit.ll
@@ -3,24 +3,24 @@
 
 define i64 @test1(i64* %ptr, i64 %val) {
 ; CHECK: test1:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: adds [[REG3:(r[0-9]?[02468])]], [[REG1]]
 ; CHECK: adc [[REG4:(r[0-9]?[13579])]], [[REG2]]
 ; CHECK: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test1:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: adds.w [[REG3:[a-z0-9]+]], [[REG1]]
 ; CHECK-THUMB: adc.w [[REG4:[a-z0-9]+]], [[REG2]]
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = atomicrmw add i64* %ptr, i64 %val seq_cst
   ret i64 %r
@@ -28,24 +28,24 @@ define i64 @test1(i64* %ptr, i64 %val) {
 
 define i64 @test2(i64* %ptr, i64 %val) {
 ; CHECK: test2:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: subs [[REG3:(r[0-9]?[02468])]], [[REG1]]
 ; CHECK: sbc [[REG4:(r[0-9]?[13579])]], [[REG2]]
 ; CHECK: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test2:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: subs.w [[REG3:[a-z0-9]+]], [[REG1]]
 ; CHECK-THUMB: sbc.w [[REG4:[a-z0-9]+]], [[REG2]]
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = atomicrmw sub i64* %ptr, i64 %val seq_cst
   ret i64 %r
@@ -53,24 +53,24 @@ define i64 @test2(i64* %ptr, i64 %val) {
 
 define i64 @test3(i64* %ptr, i64 %val) {
 ; CHECK: test3:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: and [[REG3:(r[0-9]?[02468])]], [[REG1]]
 ; CHECK: and [[REG4:(r[0-9]?[13579])]], [[REG2]]
 ; CHECK: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test3:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: and.w [[REG3:[a-z0-9]+]], [[REG1]]
 ; CHECK-THUMB: and.w [[REG4:[a-z0-9]+]], [[REG2]]
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = atomicrmw and i64* %ptr, i64 %val seq_cst
   ret i64 %r
@@ -78,24 +78,24 @@ define i64 @test3(i64* %ptr, i64 %val) {
 
 define i64 @test4(i64* %ptr, i64 %val) {
 ; CHECK: test4:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: orr [[REG3:(r[0-9]?[02468])]], [[REG1]]
 ; CHECK: orr [[REG4:(r[0-9]?[13579])]], [[REG2]]
 ; CHECK: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test4:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: orr.w [[REG3:[a-z0-9]+]], [[REG1]]
 ; CHECK-THUMB: orr.w [[REG4:[a-z0-9]+]], [[REG2]]
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = atomicrmw or i64* %ptr, i64 %val seq_cst
   ret i64 %r
@@ -103,24 +103,24 @@ define i64 @test4(i64* %ptr, i64 %val) {
 
 define i64 @test5(i64* %ptr, i64 %val) {
 ; CHECK: test5:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: eor [[REG3:(r[0-9]?[02468])]], [[REG1]]
 ; CHECK: eor [[REG4:(r[0-9]?[13579])]], [[REG2]]
 ; CHECK: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test5:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: eor.w [[REG3:[a-z0-9]+]], [[REG1]]
 ; CHECK-THUMB: eor.w [[REG4:[a-z0-9]+]], [[REG2]]
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = atomicrmw xor i64* %ptr, i64 %val seq_cst
   ret i64 %r
@@ -128,20 +128,20 @@ define i64 @test5(i64* %ptr, i64 %val) {
 
 define i64 @test6(i64* %ptr, i64 %val) {
 ; CHECK: test6:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: strexd {{[a-z0-9]+}}, {{r[0-9]?[02468]}}, {{r[0-9]?[13579]}}
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test6:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, {{[a-z0-9]+}}, {{[a-z0-9]+}}
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = atomicrmw xchg i64* %ptr, i64 %val seq_cst
   ret i64 %r
@@ -149,7 +149,7 @@ define i64 @test6(i64* %ptr, i64 %val) {
 
 define i64 @test7(i64* %ptr, i64 %val1, i64 %val2) {
 ; CHECK: test7:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: cmp [[REG1]]
 ; CHECK: cmpeq [[REG2]]
@@ -157,10 +157,10 @@ define i64 @test7(i64* %ptr, i64 %val1, i64 %val2) {
 ; CHECK: strexd {{[a-z0-9]+}}, {{r[0-9]?[02468]}}, {{r[0-9]?[13579]}}
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test7:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: cmp [[REG1]]
 ; CHECK-THUMB: it eq
@@ -169,7 +169,7 @@ define i64 @test7(i64* %ptr, i64 %val1, i64 %val2) {
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, {{[a-z0-9]+}}, {{[a-z0-9]+}}
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = cmpxchg i64* %ptr, i64 %val1, i64 %val2 seq_cst
   ret i64 %r
@@ -186,7 +186,7 @@ define i64 @test8(i64* %ptr) {
 ; CHECK: strexd {{[a-z0-9]+}}, {{r[0-9]?[02468]}}, {{r[0-9]?[13579]}}
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test8:
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
@@ -197,7 +197,7 @@ define i64 @test8(i64* %ptr) {
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, {{[a-z0-9]+}}, {{[a-z0-9]+}}
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = load atomic i64* %ptr seq_cst, align 8
   ret i64 %r
@@ -207,20 +207,20 @@ define i64 @test8(i64* %ptr) {
 ; way to write it.
 define void @test9(i64* %ptr, i64 %val) {
 ; CHECK: test9:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: strexd {{[a-z0-9]+}}, {{r[0-9]?[02468]}}, {{r[0-9]?[13579]}}
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test9:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, {{[a-z0-9]+}}, {{[a-z0-9]+}}
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   store atomic i64 %val, i64* %ptr seq_cst, align 8
   ret void
@@ -228,7 +228,7 @@ define void @test9(i64* %ptr, i64 %val) {
 
 define i64 @test10(i64* %ptr, i64 %val) {
 ; CHECK: test10:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: subs {{[a-z0-9]+}}, [[REG1]], [[REG3:(r[0-9]?[02468])]]
 ; CHECK: sbcs {{[a-z0-9]+}}, [[REG2]], [[REG4:(r[0-9]?[13579])]]
@@ -236,10 +236,10 @@ define i64 @test10(i64* %ptr, i64 %val) {
 ; CHECK: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test10:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: subs.w {{[a-z0-9]+}}, [[REG1]], [[REG3:[a-z0-9]+]]
 ; CHECK-THUMB: sbcs.w {{[a-z0-9]+}}, [[REG2]], [[REG4:[a-z0-9]+]]
@@ -247,7 +247,7 @@ define i64 @test10(i64* %ptr, i64 %val) {
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = atomicrmw min i64* %ptr, i64 %val seq_cst
   ret i64 %r
@@ -255,7 +255,7 @@ define i64 @test10(i64* %ptr, i64 %val) {
 
 define i64 @test11(i64* %ptr, i64 %val) {
 ; CHECK: test11:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: subs {{[a-z0-9]+}}, [[REG1]], [[REG3:(r[0-9]?[02468])]]
 ; CHECK: sbcs {{[a-z0-9]+}}, [[REG2]], [[REG4:(r[0-9]?[13579])]]
@@ -263,11 +263,11 @@ define i64 @test11(i64* %ptr, i64 %val) {
 ; CHECK: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 
 ; CHECK-THUMB: test11:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: subs.w {{[a-z0-9]+}}, [[REG1]], [[REG3:[a-z0-9]+]]
 ; CHECK-THUMB: sbcs.w {{[a-z0-9]+}}, [[REG2]], [[REG4:[a-z0-9]+]]
@@ -275,7 +275,7 @@ define i64 @test11(i64* %ptr, i64 %val) {
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = atomicrmw umin i64* %ptr, i64 %val seq_cst
   ret i64 %r
@@ -283,7 +283,7 @@ define i64 @test11(i64* %ptr, i64 %val) {
 
 define i64 @test12(i64* %ptr, i64 %val) {
 ; CHECK: test12:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: subs {{[a-z0-9]+}}, [[REG1]], [[REG3:(r[0-9]?[02468])]]
 ; CHECK: sbcs {{[a-z0-9]+}}, [[REG2]], [[REG4:(r[0-9]?[13579])]]
@@ -291,10 +291,10 @@ define i64 @test12(i64* %ptr, i64 %val) {
 ; CHECK: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test12:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: subs.w {{[a-z0-9]+}}, [[REG1]], [[REG3:[a-z0-9]+]]
 ; CHECK-THUMB: sbcs.w {{[a-z0-9]+}}, [[REG2]], [[REG4:[a-z0-9]+]]
@@ -302,7 +302,7 @@ define i64 @test12(i64* %ptr, i64 %val) {
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 
   %r = atomicrmw max i64* %ptr, i64 %val seq_cst
   ret i64 %r
@@ -310,7 +310,7 @@ define i64 @test12(i64* %ptr, i64 %val) {
 
 define i64 @test13(i64* %ptr, i64 %val) {
 ; CHECK: test13:
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
 ; CHECK: subs {{[a-z0-9]+}}, [[REG1]], [[REG3:(r[0-9]?[02468])]]
 ; CHECK: sbcs {{[a-z0-9]+}}, [[REG2]], [[REG4:(r[0-9]?[13579])]]
@@ -318,10 +318,10 @@ define i64 @test13(i64* %ptr, i64 %val) {
 ; CHECK: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK: cmp
 ; CHECK: bne
-; CHECK: dmb ish
+; CHECK: dmb {{ish$}}
 
 ; CHECK-THUMB: test13:
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
 ; CHECK-THUMB: ldrexd [[REG1:[a-z0-9]+]], [[REG2:[a-z0-9]+]]
 ; CHECK-THUMB: subs.w {{[a-z0-9]+}}, [[REG1]], [[REG3:[a-z0-9]+]]
 ; CHECK-THUMB: sbcs.w {{[a-z0-9]+}}, [[REG2]], [[REG4:[a-z0-9]+]]
@@ -329,7 +329,7 @@ define i64 @test13(i64* %ptr, i64 %val) {
 ; CHECK-THUMB: strexd {{[a-z0-9]+}}, [[REG3]], [[REG4]]
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
-; CHECK-THUMB: dmb ish
+; CHECK-THUMB: dmb {{ish$}}
   %r = atomicrmw umax i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
diff --git a/test/CodeGen/ARM/atomic-load-store.ll b/test/CodeGen/ARM/atomic-load-store.ll
index 66916a7c2e2..476b3ddd45d 100644
--- a/test/CodeGen/ARM/atomic-load-store.ll
+++ b/test/CodeGen/ARM/atomic-load-store.ll
@@ -6,15 +6,15 @@
 
 define void @test1(i32* %ptr, i32 %val1) {
 ; ARM: test1
-; ARM: dmb ish
+; ARM: dmb {{ish$}}
 ; ARM-NEXT: str
-; ARM-NEXT: dmb ish
+; ARM-NEXT: dmb {{ish$}}
 ; THUMBONE: test1
 ; THUMBONE: __sync_lock_test_and_set_4
 ; THUMBTWO: test1
-; THUMBTWO: dmb ish
+; THUMBTWO: dmb {{ish$}}
 ; THUMBTWO-NEXT: str
-; THUMBTWO-NEXT: dmb ish
+; THUMBTWO-NEXT: dmb {{ish$}}
   store atomic i32 %val1, i32* %ptr seq_cst, align 4
   ret void
 }
@@ -22,12 +22,12 @@ define void @test1(i32* %ptr, i32 %val1) {
 define i32 @test2(i32* %ptr) {
 ; ARM: test2
 ; ARM: ldr
-; ARM-NEXT: dmb ish
+; ARM-NEXT: dmb {{ish$}}
 ; THUMBONE: test2
 ; THUMBONE: __sync_val_compare_and_swap_4
 ; THUMBTWO: test2
 ; THUMBTWO: ldr
-; THUMBTWO-NEXT: dmb ish
+; THUMBTWO-NEXT: dmb {{ish$}}
   %val = load atomic i32* %ptr seq_cst, align 4
   ret i32 %val
 }
diff --git a/test/CodeGen/ARM/swift-atomics.ll b/test/CodeGen/ARM/swift-atomics.ll
new file mode 100644
index 00000000000..9711a9f151e
--- /dev/null
+++ b/test/CodeGen/ARM/swift-atomics.ll
@@ -0,0 +1,45 @@
+; RUN: llc -mtriple=armv7-apple-ios6.0 -mcpu=swift < %s | FileCheck %s
+; RUN: llc -mtriple=armv7-apple-ios6.0 < %s | FileCheck %s --check-prefix=CHECK-STRICT-ATOMIC
+
+; Release operations only need the store barrier provided by a "dmb ishst",
+
+define void @test_store_release(i32* %p, i32 %v) {
+; CHECK: test_store_release:
+; CHECK: dmb ishst
+; CHECK: str
+
+; CHECK-STRICT-ATOMIC: dmb {{ish$}}
+  store atomic i32 %v, i32* %p release, align 4
+  ret void
+}
+
+; However, if sequential consistency is needed *something* must ensure a release
+; followed by an acquire does not get reordered. In that case a "dmb ishst" is
+; not adequate.
+define i32 @test_seq_cst(i32* %p, i32 %v) {
+; CHECK: test_seq_cst:
+; CHECK: dmb ishst
+; CHECK: str
+; CHECK: dmb {{ish$}}
+; CHECK: ldr
+; CHECK: dmb {{ish$}}
+
+; CHECK-STRICT-ATOMIC: dmb {{ish$}}
+; CHECK-STRICT-ATOMIC: dmb {{ish$}}
+
+  store atomic i32 %v, i32* %p seq_cst, align 4
+  %val = load atomic i32* %p seq_cst, align 4
+  ret i32 %val
+}
+
+; Also, pure acquire operations should definitely not have an ishst barrier.
+
+define i32 @test_acq(i32* %addr) {
+; CHECK: test_acq:
+; CHECK: ldr
+; CHECK: dmb {{ish$}}
+
+; CHECK-STRICT-ATOMIC: dmb {{ish$}}
+  %val = load atomic i32* %addr acquire, align 4
+  ret i32 %val
+}