[LoopUnroll+LoopUnswitch] do not transform loops containing callbr

Summary:
There is currently a correctness issue when unrolling loops containing
callbr's where their indirect targets are being updated correctly to the
newly created labels, but their operands are not.  This manifests in
unrolled loops where the second and subsequent copies of callbr
instructions have blockaddresses of the label from the first instance of
the unrolled loop, which would result in nonsensical runtime control
flow.

For now, conservatively do not unroll the loop.  In the future, I think
we can pursue unrolling such loops provided we transform the cloned
callbr's operands correctly.

Such a transform and its legalities are being discussed in:
https://reviews.llvm.org/D64101

Link: https://bugs.llvm.org/show_bug.cgi?id=42489
Link: https://groups.google.com/forum/#!topic/clang-built-linux/z-hRWP9KqPI
Reviewers: fhahn, hfinkel, efriedma

Reviewed By: fhahn, hfinkel, efriedma

Subscribers: efriedma, hiraditya, zzheng, dmgreen, llvm-commits, pirama, kees, nathanchance, E5ten, craig.topper, chandlerc, glider, void, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64368

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366130 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/LoopInfo.cpp b/lib/Analysis/LoopInfo.cpp
index 00dbe30c2b3dbbc6592a025c2cade8a5d91d6173..c59e48a7a98e1b105f3199656bc40e8de19c6673 100644 (file)
--- a/lib/Analysis/LoopInfo.cpp
+++ b/lib/Analysis/LoopInfo.cpp
@@ -432,8 +432,11 @@ bool Loop::isLoopSimplifyForm() const {
 bool Loop::isSafeToClone() const {
   // Return false if any loop blocks contain indirectbrs, or there are any calls
   // to noduplicate functions.
+  // FIXME: it should be ok to clone CallBrInst's if we correctly update the
+  // operand list to reflect the newly cloned labels.
   for (BasicBlock *BB : this->blocks()) {
-    if (isa<IndirectBrInst>(BB->getTerminator()))
+    if (isa<IndirectBrInst>(BB->getTerminator()) ||
+        isa<CallBrInst>(BB->getTerminator()))
       return false;
 
     for (Instruction &I : *BB)

diff --git a/test/Transforms/LoopUnroll/callbr.ll b/test/Transforms/LoopUnroll/callbr.ll
new file mode 100644 (file)
index 0000000..22206b4
--- /dev/null
+++ b/test/Transforms/LoopUnroll/callbr.ll
@@ -0,0 +1,51 @@
+; RUN: opt -loop-unroll -S %s | FileCheck %s
+
+; Check that the loop body exists.
+; CHECK: for.body
+; CHECK: if.then
+; CHECK: asm.fallthrough
+; CHECK: l_yes
+; CHECK: for.inc
+
+; Check that the loop body does not get unrolled.  We could modify this test in
+; the future to support loop unrolling callbr's IFF we checked that the callbr
+; operands were unrolled/updated correctly, as today they are not.
+; CHECK-NOT: if.then.1
+; CHECK-NOT: asm.fallthrough.1
+; CHECK-NOT: l_yes.1
+; CHECK-NOT: for.inc.1
+; CHECK-NOT: if.then.2
+; CHECK-NOT: asm.fallthrough.2
+; CHECK-NOT: l_yes.2
+; CHECK-NOT: for.inc.2
+
+define dso_local void @d() {
+entry:
+  br label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.inc
+  ret void
+
+for.body:                                         ; preds = %for.inc, %entry
+  %e.04 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %tobool = icmp eq i32 %e.04, 0
+  br i1 %tobool, label %for.inc, label %if.then
+
+if.then:                                          ; preds = %for.body
+  callbr void asm sideeffect "1: nop\0A\09.quad b, ${0:l}, $$5\0A\09", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@d, %l_yes))
+          to label %asm.fallthrough [label %l_yes]
+
+asm.fallthrough:                                  ; preds = %if.then
+  br label %l_yes
+
+l_yes:                                            ; preds = %asm.fallthrough, %if.then
+  %call = tail call i32 (...) @g()
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body, %l_yes
+  %inc = add nuw nsw i32 %e.04, 1
+  %exitcond = icmp eq i32 %inc, 3
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+}
+
+declare dso_local i32 @g(...) local_unnamed_addr

diff --git a/test/Transforms/LoopUnswitch/callbr.ll b/test/Transforms/LoopUnswitch/callbr.ll
new file mode 100644 (file)
index 0000000..6e05374
--- /dev/null
+++ b/test/Transforms/LoopUnswitch/callbr.ll
@@ -0,0 +1,66 @@
+; RUN: opt -loop-unswitch %s -S | FileCheck %s
+
+; We want to check that the loop does not get split (so only 2 callbr's not 4).
+; It's ok to modify this test in the future should we allow the loop containing
+; callbr to be unswitched and are able to do so correctly.
+
+; CHECK: callbr void asm sideeffect "# ${0:l}", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %10))
+; CHECK: to label %7 [label %10]
+; CHECK: callbr void asm sideeffect "# ${0:l}", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %10))
+; CHECK: to label %9 [label %10]
+
+; CHECK-NOT: callbr void asm sideeffect "# ${0:l}", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %10))
+; CHECK-NOT: to label %7 [label %10]
+; CHECK-NOT: callbr void asm sideeffect "# ${0:l}", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %10))
+; CHECK-NOT: to label %9 [label %10]
+; CHECK-NOT: callbr void asm sideeffect "# ${0:l}", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %19))
+; CHECK-NOT: to label %16 [label %19]
+; CHECK-NOT: callbr void asm sideeffect "# ${0:l}", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %19))
+; CHECK-NOT: to label %18 [label %19]
+
+; This test is essentially:
+; void foo(int n) {
+;   for (int i = 0; i < 1000; ++i)
+;     if (n) {
+;       asm goto("# %l0"::::bar);
+;       bar:;
+;     } else {
+;       asm goto("# %l0"::::baz);
+;       baz:;
+;     }
+;}
+
+define dso_local void @foo(i32) #0 {
+  br label %2
+
+2:                                                ; preds = %10, %1
+  %.0 = phi i32 [ 0, %1 ], [ %11, %10 ]
+  %3 = icmp ult i32 %.0, 1000
+  br i1 %3, label %4, label %12
+
+4:                                                ; preds = %2
+  %5 = icmp eq i32 %0, 0
+  br i1 %5, label %8, label %6
+
+6:                                                ; preds = %4
+  callbr void asm sideeffect "# ${0:l}", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %10)) #0
+    to label %7 [label %10]
+
+7:                                                ; preds = %6
+  br label %10
+
+8:                                                ; preds = %4
+  callbr void asm sideeffect "# ${0:l}", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %10)) #0
+    to label %9 [label %10]
+
+9:                                                ; preds = %8
+  br label %10
+
+10:                                               ; preds = %7, %6, %9, %8
+  %11 = add nuw nsw i32 %.0, 1
+  br label %2
+
+12:                                               ; preds = %2
+  ret void
+}
+