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...other parts of the register being redefined unless the <undef> flag is set. Now, I am writing a pass the splits the following sequence of MIs MI1:: A<def> = 0xFFFFFFFF ; A is a 64bit super reg. MI2:: B<def> = C & A ; C and B are also 64bit super regs. Into NewMI_1:: B:lo_sub_reg<def> = COPY C:lo_sub_reg. NewMI_2:: B:hi_sub_reg<def> = 0 The question is how should I be setting up the <undef> flags on the def operands of NewMI_1 and 2 ? Should I set the <undef> flag only on NewMI_1 because in NewMI_2 lo_sub_reg has already been define...

...unless > the <undef> flag is set. > > Now, I am writing a pass the splits the following sequence of MIs > > MI1:: A<def> = 0xFFFFFFFF ; A is a 64bit super reg. > MI2:: B<def> = C & A ; C and B are also 64bit super regs. > > Into > NewMI_1:: B:lo_sub_reg<def> = COPY C:lo_sub_reg. > NewMI_2:: B:hi_sub_reg<def> = 0 > > The question is how should I be setting up the <undef> flags on the def > operands of NewMI_1 and 2 ? Should I set the <undef> flag only on NewMI_1 > because in NewMI_2 lo_sub_re...

Hi Jakob, Thanks for your reply. > > The <undef> flag goes on NewMI_1 because the virtual register B isn't live > before that instruction. > > But you probably shouldn't be doing this yourself. Your NewMI code isn't in > SSA form because B has multiple definitions. Just use a REG_SEQUENCE > instruction, and let the register allocator do th...