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Remove unstructured loop exits: Don't introduce loops (microsoft#6676)
Previously, if the latch exit was reachable from a different exit block for the loop, then the pass would introduce a loop involving that exit block and the latch exit. This is unwanted and unaccounted for. - Add a test for shared exits. - Add a test for non-dedicated latch exit
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‎lib/Transforms/Scalar/DxilRemoveUnstructuredLoopExits.cpp

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@@ -134,6 +134,7 @@
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/LoopUtils.h"
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@@ -466,6 +467,39 @@ static bool RemoveUnstructuredLoopExitsIteration(BasicBlock *exiting_block,
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DominatorTree *DT) {
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BasicBlock *latch = L->getLoopLatch();
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BasicBlock *latch_exit = GetExitBlockForExitingBlock(L, latch);
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// Ensure the latch-exit is "dedicated": no block outside the loop
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// branches to it.
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//
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// Suppose this iteration successfully moves an exit block X until
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// after the latch block. It will do so by rewiring the CFG so
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// the latch *exit* block will branch to X. If the latch exit
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// block is already reachable from X, then the rewiring will
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// create an unwanted loop.
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// So prevent this from happening by ensuring the latch exit is
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// "dedicated": the only branches to it come from inside the
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// loop, and hence not from X.
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//
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// The latch_exit block could have *multiple* branches to it from
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// outside the loop.
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//
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// When the edge from latch to latch_exit is split, the local picture is:
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//
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// latch --> middle --> tail
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//
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// where:
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// - Branches that used to go to latch_exit, from outside the loop, now
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// point to 'tail'.
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// - 'middle' is now an exit block for the loop, and its only incoming
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// edge is from latch.
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for (auto *pred : predecessors(latch_exit)) {
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if (!L->contains(pred)) {
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SplitEdge(latch, latch_exit, DT, LI);
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// Quit early and recalculate exit blocks.
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return true;
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}
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}
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BasicBlock *exit_block = GetExitBlockForExitingBlock(L, exiting_block);
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// If exiting block already dominates latch, then no need to do anything.

‎test/HLSL/passes/dxil_remove_unstructured_loop_exits/nondedicated-latch-exit.ll

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; RUN: opt %s -analyze -loops | FileCheck -check-prefix=LOOPBEFORE %s
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; RUN: opt %s -dxil-remove-unstructured-loop-exits -o %t.bc
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; RUN: opt %t.bc -S | FileCheck %s
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; RUN: opt %t.bc -analyze -loops | FileCheck -check-prefix=LOOPAFTER %s
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; Ensure the pass works when the latch exit is reachable from the exit block,
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; and does not introduce an extra loop.
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; If an exit block could reach the latch exit block, then an old version
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; of the pass would introduce a loop because after modification, the
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; latch-exit block would branch to the exit block (which has since been
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; repositioned in the CFG).
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;
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; entry
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; |
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; v
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; +-> header ---> then --+
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; | | | |
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; | | +---------+ |
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; | | | v
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; | v v exit0
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; +--- latch |
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; | v
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; | exit1
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; | |
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; | v
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; | exit2
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; | |
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; | +--------------+
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; | |
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; v v
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; end # 'end' is the latch-exit block
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; Before performing the loop exit restructuring, split the latch -> end edge, like this:
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;
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; entry
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; |
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; v
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; +-> header ---> then --+
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; | | | |
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; | | +---------+ |
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; | | | v
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; | v v exit0
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; +--- latch |
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; | v
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; | exit1
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; v |
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; latch.end_crit_edge v
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; | exit2
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; | |
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; | +--------------+
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; | |
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; v v
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; end
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; Then it will be safe to rewire 'then' block as follows.
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; This achieves the goal of making all exiting blocks dominate
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; the latch. And crucially, a new loop is not created.
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;
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; entry
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; |
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; v
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; +-> header ---> then
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; | | |
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; | | +---------+
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; | | |
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; | v v
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; | dx.struct.new_exiting
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; | | |
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; | v |
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; +--- latch |
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; | +-----+
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; v v
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; latch.end_crit_edge --> exit0
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; | |
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; | v
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; | exit1
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; | |
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; | v
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; | exit2
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; | |
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; | +----------------+
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; | |
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; v v
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; end
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; LOOPBEFORE: Loop at depth 1 containing: %header<header>,%then<exiting>,%latch<latch><exiting>
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; LOOPBEFORE-NOT: Loop at depth
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; Don't create an extra loop
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; LOOPAFTER-NOT: Loop at depth {{.*}} containing: {{.*}}%end
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; LOOPAFTER-NOT: Loop at depth {{.*}} containing: {{.*}}%exit0
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; LOOPAFTER-NOT: Loop at depth {{.*}} containing: {{.*}}%exit1
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; LOOPAFTER-NOT: Loop at depth {{.*}} containing: {{.*}}%exit2
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; LOOPAFTER: Loop at depth 1 containing: %header<header>,%then,%dx.struct_exit.new_exiting<exiting>,%latch<latch><exiting>
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; LOOPAFTER-NOT: Loop at depth
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target datalayout = "e-m:e-p:32:32-i1:32-i8:32-i16:32-i32:32-i64:64-f16:32-f32:32-f64:64-n8:16:32:64"
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target triple = "dxil-ms-dx"
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define void @main(i1 %cond) {
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entry:
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br label %header
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header:
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br i1 %cond, label %then, label %latch
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then:
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br i1 %cond, label %exit0, label %latch
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latch:
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br i1 %cond, label %end, label %header
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; a long chain that eventually gets to %end
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exit0:
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br label %exit1
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exit1:
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br label %exit2
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exit2:
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br label %end
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end:
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ret void
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}
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; CHECK: define void @main

‎test/HLSL/passes/dxil_remove_unstructured_loop_exits/shared-exit.ll

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; RUN: opt %s -analyze -loops | FileCheck -check-prefix=LOOPBEFORE %s
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; RUN: opt %s -dxil-remove-unstructured-loop-exits -o %t.bc
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; RUN: opt %t.bc -S | FileCheck %s
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; RUN: opt %t.bc -analyze -loops | FileCheck -check-prefix=LOOPAFTER %s
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; Two exiting blocks target the same exit block. This should work.
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; Also, the pass should not introduce a new loop, particularly not among
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; the latch-exiting blocks.
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; LOOPBEFORE: Loop at depth 1 containing: %header<header>,%then.a<exiting>,%midloop,%then.b<exiting>,%latch<latch><exiting>
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; LOOPBEFORE-NOT: Loop at depth
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; Don't create a loop containing: %end<header>,%0<exiting>,%shared_exit<latch>
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; LOOPAFTER-NOT: Loop at depth {{.*}} containing: {{.*}}%end
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; LOOPAFTER-NOT: Loop at depth {{.*}} containing: {{.*}}%0
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; LOOPAFTER-NOT: Loop at depth {{.*}} containing: {{.*}}%shared_exit
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; LOOPAFTER: Loop at depth 1 containing: %header<header>,%then.a,%dx.struct_exit.new_exiting<exiting>,%midloop,%then.b,%dx.struct_exit.new_exiting2<exiting>,%latch<latch><exiting>
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; LOOPAFTER-NOT: Loop at depth
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target datalayout = "e-m:e-p:32:32-i1:32-i8:32-i16:32-i32:32-i64:64-f16:32-f32:32-f64:64-n8:16:32:64"
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target triple = "dxil-ms-dx"
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; The input has two if-then-else structures in a row:
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; header/then.a/midloop
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; midloop/then.b/latch
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;
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; entry
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; |
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; v
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; +-> header ---> then.a ------+
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; | | | |
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; | | +-------+ |
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; | v v v
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; | midloop --> then.b --> shared_exit
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; | | | |
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; | | +-------+ |
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; | v v |
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; +--- latch |
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; | |
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; | +--------------------+
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; | |
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; v v
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; end
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define void @main(i1 %cond) {
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entry:
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br label %header
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header:
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br i1 %cond, label %then.a, label %midloop
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then.a:
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br i1 %cond, label %shared_exit, label %midloop
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midloop:
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br i1 %cond, label %then.b, label %latch
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then.b:
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br i1 %cond, label %shared_exit, label %latch
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latch:
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br i1 %cond, label %end, label %header
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shared_exit:
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br label %end
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end:
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ret void
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}
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; CHECK: define void @main

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