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[mlir][scf] Remove identical scf.for iter args #127145

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Merged
merged 1 commit into from
Feb 14, 2025
Merged

[mlir][scf] Remove identical scf.for iter args #127145

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29 changes: 24 additions & 5 deletions mlir/lib/Dialect/SCF/IR/SCF.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -843,9 +843,8 @@ namespace {
// 3) The iter arguments have no use and the corresponding (operation) results
// have no use.
//
// These arguments must be defined outside of
// the ForOp region and can just be forwarded after simplifying the op inits,
// yields and returns.
// These arguments must be defined outside of the ForOp region and can just be
// forwarded after simplifying the op inits, yields and returns.
//
// The implementation uses `inlineBlockBefore` to steal the content of the
// original ForOp and avoid cloning.
Expand All @@ -871,6 +870,7 @@ struct ForOpIterArgsFolder : public OpRewritePattern<scf::ForOp> {
newIterArgs.reserve(forOp.getInitArgs().size());
newYieldValues.reserve(numResults);
newResultValues.reserve(numResults);
DenseMap<std::pair<Value, Value>, std::pair<Value, Value>> initYieldToArg;
for (auto [init, arg, result, yielded] :
llvm::zip(forOp.getInitArgs(), // iter from outside
forOp.getRegionIterArgs(), // iter inside region
Expand All @@ -884,13 +884,32 @@ struct ForOpIterArgsFolder : public OpRewritePattern<scf::ForOp> {
// result has no use.
bool forwarded = (arg == yielded) || (init == yielded) ||
(arg.use_empty() && result.use_empty());
keepMask.push_back(!forwarded);
canonicalize |= forwarded;
if (forwarded) {
canonicalize = true;
keepMask.push_back(false);
newBlockTransferArgs.push_back(init);
newResultValues.push_back(init);
continue;
}

// Check if a previous kept argument always has the same values for init
// and yielded values.
if (auto it = initYieldToArg.find({init, yielded});
it != initYieldToArg.end()) {
canonicalize = true;
keepMask.push_back(false);
auto [sameArg, sameResult] = it->second;
rewriter.replaceAllUsesWith(arg, sameArg);
rewriter.replaceAllUsesWith(result, sameResult);
// The replacement value doesn't matter because there are no uses.
newBlockTransferArgs.push_back(init);
newResultValues.push_back(init);
continue;
}

// This value is kept.
initYieldToArg.insert({{init, yielded}, {arg, result}});
keepMask.push_back(true);
newIterArgs.push_back(init);
newYieldValues.push_back(yielded);
newBlockTransferArgs.push_back(Value()); // placeholder with null value
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18 changes: 18 additions & 0 deletions mlir/test/Dialect/SCF/canonicalize.mlir
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Expand Up @@ -821,6 +821,24 @@ func.func @fold_away_iter_and_result_with_no_use(%arg0 : i32,

// -----

// CHECK-LABEL: @replace_duplicate_iter_args
// CHECK-SAME: [[LB:%arg[0-9]]]: index, [[UB:%arg[0-9]]]: index, [[STEP:%arg[0-9]]]: index, [[A:%arg[0-9]]]: index, [[B:%arg[0-9]]]: index
func.func @replace_duplicate_iter_args(%lb: index, %ub: index, %step: index, %a: index, %b: index) -> (index, index, index, index) {
// CHECK-NEXT: [[RES:%.*]]:2 = scf.for {{.*}} iter_args([[K0:%.*]] = [[A]], [[K1:%.*]] = [[B]])
%0:4 = scf.for %i = %lb to %ub step %step iter_args(%k0 = %a, %k1 = %b, %k2 = %b, %k3 = %a) -> (index, index, index, index) {
// CHECK-NEXT: [[V0:%.*]] = arith.addi [[K0]], [[K1]]
%1 = arith.addi %k0, %k1 : index
// CHECK-NEXT: [[V1:%.*]] = arith.addi [[K1]], [[K0]]
%2 = arith.addi %k2, %k3 : index
// CHECK-NEXT: yield [[V0]], [[V1]]
scf.yield %1, %2, %2, %1 : index, index, index, index
}
// CHECK: return [[RES]]#0, [[RES]]#1, [[RES]]#1, [[RES]]#0
return %0#0, %0#1, %0#2, %0#3 : index, index, index, index
}

// -----

func.func private @do(%arg0: tensor<?x?xf32>) -> tensor<?x?xf32>

func.func @matmul_on_tensors(%t0: tensor<32x1024xf32>) -> tensor<?x?xf32> {
Expand Down