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<div style="font-family: Aptos; font-size: 12pt; color: rgb(0, 0, 0);"><span style="text-transform: none;">Hi All</span></div>
<div style="font-family: Aptos; font-size: 12pt; color: rgb(0, 0, 0);">I am working on JDK-8216554 — optimizing constant loading on PPC64 by replacing the two-instruction sequence (ADDIS + LD) with a single LD instruction when the method TOC offset fits in
a 16-bit signed displacement.</div>
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<b>Background:</b></div>
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Currently on PPC64, all long constant loads in C2-compiled code go through a two-node hi/lo pair (loadConL_hi + loadConL_lo), emitting ADDIS + LD regardless of the constant pool offset. This is because the decision was hardcoded as:</div>
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const bool large_constant_pool = true; // TODO: PPC port C->cfg()->_consts_size > 4000;</div>
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The TODO was never implemented.</div>
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The intent was to check the constant pool size at postalloc time and choose between a single-node (small) or two-node (large) path. However, the constant pool size is not available at postalloc — constant_table().size() returns 0 on PPC64 because the platform
uses its own TOC-based constant pool via MacroAssembler::long_constant(), bypassing the generic C2 ConstantTable.</div>
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Approach:</div>
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Thanks to Dean Long's suggestion of creating a single smart node that decides its instruction encoding at emit time, I create a single loadConLNode at postalloc and defer the instruction selection to the emit stage. During emit, the method TOC offset (toc_offset)
for each constant is known and stable, so the decision is made per-constant:</div>
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int toc_offset = __ offset_to_method_toc(const_toc_addr);</div>
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if (Assembler::is_simm(toc_offset, 16)) {</div>
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// Small path: single LD + NOP (8 bytes)</div>
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__ ld(dst, toc_offset, toc_reg);</div>
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__ nop();</div>
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} else {</div>
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// Large path: ADDIS + LD (8 bytes)</div>
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__ addis(dst, toc_reg, largeoffset_si16_si16_hi(toc_offset));</div>
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__ ld(dst, largeoffset_si16_si16_lo(toc_offset), dst);</div>
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}</div>
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Both paths emit exactly 8 bytes, matching the scratch emit reservation. The NOP in the small path maintains size consistency.</div>
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One thing I would like confirmation on: is it correct to rely on the method TOC offset (toc_offset) at emit time for the instruction selection? </div>
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I want to confirm there are no edge cases where this assumption breaks.</div>
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I would appreciate any feedback on this approach.</div>
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Thanks,</div>
<div style="font-family: Aptos; font-size: 12pt; color: rgb(0, 0, 0);"><span style="text-transform: none;">Suchismith Roy</span></div>
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