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[LoopUnroll+LoopUnswitch] do not transform loops containing callbr

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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

llvm-svn: 366130
This commit is contained in:
Nick Desaulniers 2019-07-15 21:16:29 +00:00
parent f6ccac1c17
commit fe3275edf3
3 changed files with 121 additions and 1 deletions
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5
lib/Analysis/LoopInfo.cpp
View File

@ -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)

51
test/Transforms/LoopUnroll/callbr.ll Normal file
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@ -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

66
test/Transforms/LoopUnswitch/callbr.ll Normal file
View File

@ -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
}