GlobalISel: Fix defined register of invariant.start (#125664) In contrast to SelectionDAG, GlobalISel created a new virtual register for the return value of invariant.start, leaving subsequent users of the invariant.start value with an undefined reference. A minimal example: ``` %tmp = alloca i32, align 4, addrspace(5) %tmpI = call ptr @llvm.invariant.start.p5(i64 4, ptr addrspace(5) %tmp) #3 call void @llvm.invariant.end.p5(ptr %tmpI, i64 4, ptr addrspace(5) %tmp) #3 store i32 %i, ptr %tmpI, align 4 ``` Although the return value of invariant.start might not be intended for any use beyond invariant.end (the fuzzer might not have created a sensible situation here), an implicit definition of the corresponding virtual register avoids a segfault in the target instruction selector later. This LLVM defect was identified via the AMD Fuzzing project.
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