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<p class="MsoNormal"><span style="color:black">Hi all,</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">I’m just wondering what the best practices are in terms of choosing registers.</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">I was looking at the assembly code for tlab allocation (<a href="https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L131" title="https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L131"><span style="color:#044A91">https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L131</span></a>)
and noticed the following.</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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tmp1 register is invalid.<span style="font-size:12.0pt"><o:p></o:p></span></li><li class="MsoListParagraph" style="color:black;margin-top:0cm;margin-bottom:0cm;mso-list:l0 level1 lfo3">
tmp2 register is valid but sometimes clashes with var_size_in_bytes, which requires the workaround here<span class="apple-converted-space"> </span><a href="https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L155" title="https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L155"><span style="color:#044A91">https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L155</span></a><span style="font-size:12.0pt"><o:p></o:p></span></li><li class="MsoListParagraph" style="color:black;margin-top:0cm;margin-bottom:0cm;mso-list:l0 level1 lfo3">
t0 is used as a temporary register despite not passed into the method as a temporary register<span class="apple-converted-space"> </span><a href="https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L149" title="https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L149"><span style="color:#044A91">https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/gc/shared/barrierSetAssembler_riscv.cpp#L149</span></a><span style="font-size:12.0pt"><o:p></o:p></span></li></ol>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">I’d really appreciate if someone could shed insight on how these decisions are made so that I can avoid common pitfalls when writing assembly code. The only resource I found at the moment is the architecture description file<span class="apple-converted-space"> </span><a href="https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/riscv.ad#L73" title="https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/riscv.ad#L73"><span style="color:#044A91">https://github.com/openjdk/jdk/blob/5a9cd33632862aa2249794902d4168a7fe143054/src/hotspot/cpu/riscv/riscv.ad#L73</span></a><span class="apple-converted-space"> </span>which
suggests x5(t0)-x6(t1) can always be used as temporary registers.</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span lang="EN-US" style="color:black">Sincerely,</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span lang="EN-US" style="color:black">Zixian</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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