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RISC-V: elide unnecessary sign extend when expanding cmp_and_jump
RV64 compare and branch instructions only support 64-bit operands. At Expand time, the backend conservatively zero/sign extends its operands even if not needed, such as incoming 32-bit function args which ABI/ISA guarantee to be sign-extended already. And subsequently REE fails to eliminate them as "missing defintion(s)" or "multiple definition(s) since function args don't have explicit definition. So during expand riscv_extend_comparands (), if an operand is a subreg-promoted SI with inner DI, which is representative of a function arg, just peel away the subreg to expose the DI, eliding the sign extension. As Jeff noted this routine is also used in if-conversion so also helps there. Note there's currently patches floating around to improve REE and also a new pass to eliminate unneccesary extensions, but it is still beneficial to not generate those extra extensions in first place. It is obviously less work for post-reload passes such as REE, but even for earlier passes, such as combine, having to deal with one less thing and ensuing fewer combinations is a win too. Way too many existing tests used to observe this issue. e.g. gcc.c-torture/compile/20190827-1.c -O2 -march=rv64gc It elimiates the SEXT.W Tested with rv64gc with no regressions, I'm relying on PAtrick's pre-commit CI to do the full testing. gcc/ChangeLog: * config/riscv/riscv.cc (riscv_sign_extend_if_not_subreg_prom): New. * (riscv_extend_comparands): Call New function on operands. Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>
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