[LoopUnswitch] Unswitch on conditions feeding into guards
Summary:
This is a straightforward extension of what LoopUnswitch does to
branches to guards. That is, we unswitch
```
for (;;) {
...
guard(loop_invariant_cond);
...
}
```
into
```
if (loop_invariant_cond) {
for (;;) {
...
// There is no need to emit guard(true)
...
}
} else {
for (;;) {
...
guard(false);
// SimplifyCFG will clean this up by adding an
// unreachable after the guard(false)
...
}
}
```
Reviewers: majnemer
Subscribers: mcrosier, llvm-commits, mzolotukhin
Differential Revision: http://reviews.llvm.org/D21725
llvm-svn: 273801
This commit is contained in:
Sanjoy Das
parent
7dda0edb5f
commit
a37bb4a65d
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@ -65,6 +65,7 @@ using namespace llvm;
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STATISTIC(NumBranches, "Number of branches unswitched");
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STATISTIC(NumSwitches, "Number of switches unswitched");
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STATISTIC(NumGuards, "Number of guards unswitched");
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STATISTIC(NumSelects , "Number of selects unswitched");
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STATISTIC(NumTrivial , "Number of unswitches that are trivial");
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STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
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@ -514,22 +515,34 @@ bool LoopUnswitch::processCurrentLoop() {
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return true;
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}
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// Do not unswitch loops containing convergent operations, as we might be
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// making them control dependent on the unswitch value when they were not
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// before.
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// FIXME: This could be refined to only bail if the convergent operation is
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// not already control-dependent on the unswitch value.
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// Run through the instructions in the loop, keeping track of three things:
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//
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// - That we do not unswitch loops containing convergent operations, as we
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// might be making them control dependent on the unswitch value when they
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// were not before.
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// FIXME: This could be refined to only bail if the convergent operation is
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// not already control-dependent on the unswitch value.
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//
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// - That basic blocks in the loop contain invokes whose predecessor edges we
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// cannot split.
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//
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// - The set of guard intrinsics encountered (these are non terminator
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// instructions that are also profitable to be unswitched).
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SmallVector<IntrinsicInst *, 4> Guards;
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for (const auto BB : currentLoop->blocks()) {
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for (auto &I : *BB) {
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auto CS = CallSite(&I);
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if (!CS) continue;
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if (CS.hasFnAttr(Attribute::Convergent))
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return false;
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// Return false if any loop blocks contain invokes whose predecessor edges
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// we cannot split.
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if (auto *II = dyn_cast<InvokeInst>(&I))
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if (!II->getUnwindDest()->canSplitPredecessors())
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return false;
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if (auto *II = dyn_cast<IntrinsicInst>(&I))
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if (II->getIntrinsicID() == Intrinsic::experimental_guard)
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Guards.push_back(II);
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}
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}
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@ -549,6 +562,19 @@ bool LoopUnswitch::processCurrentLoop() {
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return false;
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}
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for (IntrinsicInst *Guard : Guards) {
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Value *LoopCond =
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FindLIVLoopCondition(Guard->getOperand(0), currentLoop, Changed);
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if (LoopCond &&
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UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context))) {
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// NB! Unswitching (if successful) could have erased some of the
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// instructions in Guards leaving dangling pointers there. This is fine
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// because we're returning now, and won't look at Guards again.
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++NumGuards;
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return true;
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}
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}
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// Loop over all of the basic blocks in the loop. If we find an interior
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// block that is branching on a loop-invariant condition, we can unswitch this
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// loop.
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@ -0,0 +1,97 @@
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; RUN: opt -S -loop-unswitch < %s | FileCheck %s
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declare void @llvm.experimental.guard(i1, ...)
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define void @f_0(i32 %n, i32* %ptr, i1 %c) {
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; CHECK-LABEL: @f_0(
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; CHECK: loop.us:
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; CHECK-NOT: guard
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; CHECK: loop:
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; CHECK: call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
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entry:
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br label %loop
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loop:
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%iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
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%iv.inc = add i32 %iv, 1
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call void(i1, ...) @llvm.experimental.guard(i1 %c) [ "deopt"() ]
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store volatile i32 0, i32* %ptr
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%becond = icmp ult i32 %iv.inc, %n
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br i1 %becond, label %leave, label %loop
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leave:
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ret void
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}
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define void @f_1(i32 %n, i32* %ptr, i1 %c_0, i1 %c_1) {
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; CHECK-LABEL: @f_1(
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; CHECK: loop.us.us:
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; CHECK-NOT: guard
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; CHECK: loop.us:
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; CHECK: call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"(i32 2) ]
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; CHECK-NOT: guard
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; CHECK: loop.us1:
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; CHECK: call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"(i32 1) ]
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; CHECK-NOT: guard
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; CHECK: loop:
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; CHECK: call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"(i32 1) ]
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; CHECK: call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"(i32 2) ]
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entry:
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br label %loop
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loop:
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%iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
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%iv.inc = add i32 %iv, 1
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call void(i1, ...) @llvm.experimental.guard(i1 %c_0) [ "deopt"(i32 1) ]
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store volatile i32 0, i32* %ptr
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call void(i1, ...) @llvm.experimental.guard(i1 %c_1) [ "deopt"(i32 2) ]
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%becond = icmp ult i32 %iv.inc, %n
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br i1 %becond, label %leave, label %loop
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leave:
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ret void
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}
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; Basic negative test
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define void @f_3(i32 %n, i32* %ptr, i1* %c_ptr) {
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; CHECK-LABEL: @f_3(
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; CHECK-NOT: loop.us:
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entry:
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br label %loop
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loop:
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%iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
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%iv.inc = add i32 %iv, 1
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%c = load volatile i1, i1* %c_ptr
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call void(i1, ...) @llvm.experimental.guard(i1 %c) [ "deopt"() ]
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store volatile i32 0, i32* %ptr
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%becond = icmp ult i32 %iv.inc, %n
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br i1 %becond, label %leave, label %loop
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leave:
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ret void
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}
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define void @f_4(i32 %n, i32* %ptr, i1 %c) {
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; CHECK-LABEL: @f_4(
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;
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; Demonstrate that unswitching on one guard can cause another guard to
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; be erased (this has implications on what guards we can keep raw
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; pointers to).
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entry:
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br label %loop
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loop:
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%iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
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%iv.inc = add i32 %iv, 1
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call void(i1, ...) @llvm.experimental.guard(i1 %c) [ "deopt"(i32 1) ]
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store volatile i32 0, i32* %ptr
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%neg = xor i1 %c, 1
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call void(i1, ...) @llvm.experimental.guard(i1 %neg) [ "deopt"(i32 2) ]
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%becond = icmp ult i32 %iv.inc, %n
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br i1 %becond, label %leave, label %loop
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leave:
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ret void
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}
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