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Add support for register-based IBF and EBF #29
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mbitsnbites
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Jun 20, 2023
Hi all, We noticed that calls to the vadcq and vsbcq intrinsics, both of which use __builtin_arm_set_fpscr_nzcvqc to set the Carry flag in the FPSCR, would produce the following code: ``` < r2 is the *carry input > vmrs r3, FPSCR_nzcvqc bic r3, r3, #536870912 orr r3, r3, r2, lsl #29 vmsr FPSCR_nzcvqc, r3 ``` when the MVE ACLE instead gives a different instruction sequence of: ``` < Rt is the *carry input > VMRS Rs,FPSCR_nzcvqc BFI Rs,Rt,#29,#1 VMSR FPSCR_nzcvqc,Rs ``` the bic + orr pair is slower and it's also wrong, because, if the *carry input is greater than 1, then we risk overwriting the top two bits of the FPSCR register (the N and Z flags). This turned out to be a problem in the header file and the solution was to simply add a `& 1x0u` to the `*carry` input: then the compiler knows that we only care about the lowest bit and can optimise to a BFI. Ok for trunk? Thanks, Stam Markianos-Wright gcc/ChangeLog: * config/arm/arm_mve.h (__arm_vadcq_s32): Fix arithmetic. (__arm_vadcq_u32): Likewise. (__arm_vadcq_m_s32): Likewise. (__arm_vadcq_m_u32): Likewise. (__arm_vsbcq_s32): Likewise. (__arm_vsbcq_u32): Likewise. (__arm_vsbcq_m_s32): Likewise. (__arm_vsbcq_m_u32): Likewise. * config/arm/mve.md (get_fpscr_nzcvqc): Make unspec_volatile. gcc/testsuite/ChangeLog: * gcc.target/arm/mve/mve_vadcq_vsbcq_fpscr_overwrite.c: New.
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In register-based
IBF/EBF, the<offset:width>arguments are packed in a register as follows:Use an
IBF(orPACK.H?) instruction to pack the arguments into a single register before issuing the intended instruction (i.e. only two instructions in total).See
define_insn "*ibfsi3"and friends inmrisc32.mdfor the current immediate-based versions.Example C++ code: https://godbolt.org/z/6GWzq8hxa
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