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[MLIR][SCF] Propagate loop annotation during while op lowering #151746

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merged 1 commit into from
Aug 1, 2025
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[MLIR][SCF] Propagate loop annotation during while op lowering #151746

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37 changes: 21 additions & 16 deletions mlir/lib/Conversion/SCFToControlFlow/SCFToControlFlow.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -312,6 +312,19 @@ struct ForallLowering : public OpRewritePattern<mlir::scf::ForallOp> {

} // namespace

static void propagateLoopAttrs(Operation *scfOp, Operation *brOp) {
// Let the CondBranchOp carry the LLVM attributes from the ForOp, such as the
// llvm.loop_annotation attribute.
// LLVM requires the loop metadata to be attached on the "latch" block. Which
// is the back-edge to the header block (conditionBlock)
SmallVector<NamedAttribute> llvmAttrs;
llvm::copy_if(scfOp->getAttrs(), std::back_inserter(llvmAttrs),
[](auto attr) {
return isa<LLVM::LLVMDialect>(attr.getValue().getDialect());
});
brOp->setDiscardableAttrs(llvmAttrs);
}

LogicalResult ForLowering::matchAndRewrite(ForOp forOp,
PatternRewriter &rewriter) const {
Location loc = forOp.getLoc();
Expand Down Expand Up @@ -350,17 +363,7 @@ LogicalResult ForLowering::matchAndRewrite(ForOp forOp,
auto branchOp =
cf::BranchOp::create(rewriter, loc, conditionBlock, loopCarried);

// Let the CondBranchOp carry the LLVM attributes from the ForOp, such as the
// llvm.loop_annotation attribute.
// LLVM requires the loop metadata to be attached on the "latch" block. Which
// is the back-edge to the header block (conditionBlock)
SmallVector<NamedAttribute> llvmAttrs;
llvm::copy_if(forOp->getAttrs(), std::back_inserter(llvmAttrs),
[](auto attr) {
return isa<LLVM::LLVMDialect>(attr.getValue().getDialect());
});
branchOp->setDiscardableAttrs(llvmAttrs);

propagateLoopAttrs(forOp, branchOp);
rewriter.eraseOp(terminator);

// Compute loop bounds before branching to the condition.
Expand Down Expand Up @@ -589,9 +592,10 @@ LogicalResult WhileLowering::matchAndRewrite(WhileOp whileOp,

rewriter.setInsertionPointToEnd(after);
auto yieldOp = cast<scf::YieldOp>(after->getTerminator());
rewriter.replaceOpWithNewOp<cf::BranchOp>(yieldOp, before,
yieldOp.getResults());
auto latch = rewriter.replaceOpWithNewOp<cf::BranchOp>(yieldOp, before,
yieldOp.getResults());

propagateLoopAttrs(whileOp, latch);
// Replace the op with values "yielded" from the "before" region, which are
// visible by dominance.
rewriter.replaceOp(whileOp, args);
Expand Down Expand Up @@ -631,10 +635,11 @@ DoWhileLowering::matchAndRewrite(WhileOp whileOp,
// Loop around the "before" region based on condition.
rewriter.setInsertionPointToEnd(before);
auto condOp = cast<ConditionOp>(before->getTerminator());
cf::CondBranchOp::create(rewriter, condOp.getLoc(), condOp.getCondition(),
before, condOp.getArgs(), continuation,
ValueRange());
auto latch = cf::CondBranchOp::create(
rewriter, condOp.getLoc(), condOp.getCondition(), before,
condOp.getArgs(), continuation, ValueRange());

propagateLoopAttrs(whileOp, latch);
// Replace the op with values "yielded" from the "before" region, which are
// visible by dominance.
rewriter.replaceOp(whileOp, condOp.getArgs());
Expand Down
43 changes: 42 additions & 1 deletion mlir/test/Conversion/SCFToControlFlow/convert-to-cfg.mlir
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Expand Up @@ -708,4 +708,45 @@ func.func @simple_std_for_loops_annotation(%arg0 : index, %arg1 : index, %arg2 :
} {llvm.loop_annotation = #full_unroll}
} {llvm.loop_annotation = #no_unroll}
return
}
}

// -----

// CHECK: #[[LOOP_UNROLL_DISABLE:.*]] = #llvm.loop_unroll<disable = true>
// CHECK: #[[NO_UNROLL:.*]] = #llvm.loop_annotation<unroll = #[[LOOP_UNROLL_DISABLE]]>
// CHECK: func @simple_while_loops_annotation
// CHECK: cf.br
// CHECK: cf.cond_br {{.*}} {llvm.loop_annotation = #[[NO_UNROLL]]}
// CHECK: return
#no_unroll = #llvm.loop_annotation<unroll = <disable = true>>
func.func @simple_while_loops_annotation(%arg0 : i1) {
scf.while : () -> () {
scf.condition(%arg0)
} do {
scf.yield
} attributes {llvm.loop_annotation = #no_unroll}
return
}

// -----

// CHECK: #[[LOOP_UNROLL_DISABLE:.*]] = #llvm.loop_unroll<disable = true>
// CHECK: #[[NO_UNROLL:.*]] = #llvm.loop_annotation<unroll = #[[LOOP_UNROLL_DISABLE]]>
// CHECK: func @do_while_loops_annotation
// CHECK: cf.br
// CHECK: cf.cond_br
// CHECK: cf.br {{.*}} {llvm.loop_annotation = #[[NO_UNROLL]]}
// CHECK: return
#no_unroll = #llvm.loop_annotation<unroll = <disable = true>>
func.func @do_while_loops_annotation() {
%c0_i32 = arith.constant 0 : i32
scf.while (%arg2 = %c0_i32) : (i32) -> (i32) {
%0 = "test.make_condition"() : () -> i1
scf.condition(%0) %c0_i32 : i32
} do {
^bb0(%arg2: i32):
scf.yield %c0_i32: i32
} attributes {llvm.loop_annotation = #no_unroll}
return
}