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<p class="MsoNormal"><span style="mso-ligatures:all">Hi LLVM developers.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all">Working with GC pointers (<a href="https://llvm.org/docs/LangRef.html#accurate-garbage-collection-intrinsics">https://llvm.org/docs/LangRef.html#accurate-garbage-collection-intrinsics</a>) there can be a
need for some optimizations to get pointer base and/or offset from the base. The base can be calculated by generating needed instructions as it is done by the RewriteStatepointsForGC::findBasePointer() function. The offset can be calculated in the same way.
Though to not expose the base calculation and to make the offset calculation as simple as (ptrtoint(derived_ptr) - ptrtoint(base_ptr)), which is illegal outside RewriteStatepointsForGC, 2 intrinsics are proposed:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all"> declare i8 addrspace(1)* @llvm.experimental.gc.get.pointer.base(i8 addrspace(1)* readnone nocapture %base_ptr) nounwind readnone willreturn "gc-leaf-function"<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all"> declare i64 @llvm.experimental.gc.get.pointer.offset(i8 addrspace(1)* readnone nocapture %base_ptr) nounwind readnone willreturn "gc-leaf-function"<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all">These intrinsics are inlined by RewriteStatepointsForGC along with generation of statepoint sequences.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all">For example, with these new intrinsics the GC parseable lowering for atomic memcpy intrinsics (<a href="https://reviews.llvm.org/D88861">https://reviews.llvm.org/D88861</a>) could be implemented as a separate
pass.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all">Here is a patch that implements the proposal.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all"><a href="https://reviews.llvm.org/D100445">https://reviews.llvm.org/D100445</a> [RS4GC] Introduce intrinsics to get base ptr and offset<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-ligatures:all"><o:p> </o:p></span></p>
<p class="MsoNormal">Thanks.<o:p></o:p></p>
<p class="MsoNormal">-Yevgeny Rouban<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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