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8381872: C2: assert before block local scheduling failed with UseCompactObjectHeaders #30762

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8381872: C2: assert before block local scheduling failed with UseCompactObjectHeaders #30762

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c2532c8
8381872: C2: assert before block local scheduling failed with UseComp…
rkennke Apr 16, 2026
925e9f4
Relax IR rules in test to allow DecodeN when peephole bails out
rkennke Apr 16, 2026
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25 changes: 24 additions & 1 deletion src/hotspot/cpu/x86/peephole_x86_64.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -338,6 +338,29 @@ bool Peephole::lea_remove_redundant(Block* block, int block_index, PhaseCFG* cfg
}

// We now have verified that the decode is redundant and can be removed with a peephole.

// Before rewiring, check that the new base won't create a register conflict.
// The new base (lea_address) might share a register with a dependant_lea.
// If the dependant_lea overwrites that register and the base is still live
// (used in successor blocks), this creates a conflict detected by verify_good_schedule.
Node* new_base = lea_derived_oop->in(AddPNode::Address);
OptoReg::Name new_base_reg = ra_->get_reg_first(new_base);
if (OptoReg::is_valid(new_base_reg)) {
for (DUIterator_Fast imax, i = decode->fast_outs(imax); i < imax; i++) {
Node* dependant_lea = decode->fast_out(i);
if (dependant_lea->is_Mach() && dependant_lea->as_Mach()->ideal_Opcode() == Op_AddP) {
OptoReg::Name lea_reg = ra_->get_reg_first(dependant_lea);
OptoReg::Name lea_reg2 = ra_->get_reg_second(dependant_lea);
// If the new base register overlaps with any register slot of
// the dependant lea's output, the rewiring would create a conflict
// when the lea overwrites the base pointer's register.
if (new_base_reg == lea_reg || new_base_reg == lea_reg2) {
return false;
}
}
}
}

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@dean-long dean-long Apr 21, 2026

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I experimented with this so I could understand it better, and something seems to be off. The above code disables the optimization even if the only dependant_lea is the lone lea_derived_oop, in which case it would seem to be OK because we aren't replacing the base with a "new base".
Also, I tried disabling the above change, and the test below still passes.

// Remove the projection
block->find_remove(proj);
cfg_->map_node_to_block(proj, nullptr);
Expand All @@ -346,7 +369,7 @@ bool Peephole::lea_remove_redundant(Block* block, int block_index, PhaseCFG* cfg
for (DUIterator_Fast imax, i = decode->fast_outs(imax); i < imax; i++) {
Node* dependant_lea = decode->fast_out(i);
if (dependant_lea->is_Mach() && dependant_lea->as_Mach()->ideal_Opcode() == Op_AddP) {
dependant_lea->set_req(AddPNode::Base, lea_derived_oop->in(AddPNode::Address));
dependant_lea->set_req(AddPNode::Base, new_base);
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@mhaessig mhaessig Apr 24, 2026 •
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Is it necessary that the base input of a leaP... is non-null? Given that the leaP... does not appear in an oop map, can we make the base input nullptr, so we don't need to worry about register conflict anymore?

You are absolutely right, @merykitty. The AddPNode::Base is only for the oop map and does not even get matched. This peephole is for cleaning up decodes that nevver get used in an oop map so rewiring is not even necessary. I wish I had thought of this a year ago...

The following does the trick (inspired by AddPNode::make_off_heap since using nullptr does not work):

Suggested change
dependant_lea->set_req(AddPNode::Base, new_base);
dependant_lea->set_req(AddPNode::Base, Compile::current()->top());

By not introducing a dependency, there will also not be a conflict.

// This deleted something in the out array, hence adjust i, imax.
--i;
--imax;
Expand Down
17 changes: 11 additions & 6 deletions test/hotspot/jtreg/compiler/codegen/TestRedundantLea.java
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Expand Up @@ -298,8 +298,11 @@ class StoreNTest {
@IR(counts = {IRNode.DECODE_HEAP_OOP_NOT_NULL, "=2"},
phase = {CompilePhase.FINAL_CODE},
applyIf = {"OptoPeephole", "false"})
// Test that the peephole worked for leaPCompressedOopOffset
@IR(failOn = {IRNode.DECODE_HEAP_OOP_NOT_NULL},
// Test that the peephole worked for leaPCompressedOopOffset.
// The peephole may bail out if the new base pointer's register
// would conflict with a dependent lea's output register, so we
// allow up to the non-peephole count.
@IR(counts = {IRNode.DECODE_HEAP_OOP_NOT_NULL, "<=2"},
phase = {CompilePhase.FINAL_CODE},
applyIf = {"OptoPeephole", "true"})
// Test that the peephole removes a spill.
Expand Down Expand Up @@ -340,8 +343,9 @@ public void testPhiSpill() {
@IR(counts = {IRNode.DECODE_HEAP_OOP_NOT_NULL, "=2"},
phase = {CompilePhase.FINAL_CODE},
applyIf = {"OptoPeephole", "false"})
// Test that the peephole worked for leaPCompressedOopOffset
@IR(failOn = {IRNode.DECODE_HEAP_OOP_NOT_NULL},
// Test that the peephole worked for leaPCompressedOopOffset.
// The peephole may bail out due to register conflicts.
@IR(counts = {IRNode.DECODE_HEAP_OOP_NOT_NULL, "<=2"},
phase = {CompilePhase.FINAL_CODE},
applyIf = {"OptoPeephole", "true"})
public void testNoAlloc() {
Expand All @@ -357,8 +361,9 @@ public void testNoAlloc() {
@IR(counts = {IRNode.DECODE_HEAP_OOP_NOT_NULL, "=1"},
phase = {CompilePhase.FINAL_CODE},
applyIf = {"OptoPeephole", "false"})
// Test that the peephole worked for leaPCompressedOopOffset
@IR(failOn = {IRNode.DECODE_HEAP_OOP_NOT_NULL},
// Test that the peephole worked for leaPCompressedOopOffset.
// The peephole may bail out due to register conflicts.
@IR(counts = {IRNode.DECODE_HEAP_OOP_NOT_NULL, "<=1"},
phase = {CompilePhase.FINAL_CODE},
applyIf = {"OptoPeephole", "true"})
public void testNoAllocSameArray() {
Expand Down