> On 30 Aug 2017, at 9:43 PM, Michael Clark <michaeljcl...@mac.com> wrote: > >>> diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c >>> index ce632ae..25dd70f 100644 >>> --- a/gcc/simplify-rtx.c >>> +++ b/gcc/simplify-rtx.c >>> @@ -1503,6 +1503,10 @@ simplify_unary_operation_1 (enum rtx_code code, >>> machine_mode mode, rtx op) >>> /* (sign_extend:M (lshiftrt:N <X> (const_int I))) is better as >>> (zero_extend:M (lshiftrt:N <X> (const_int I))) if I is not 0. */ >>> if (GET_CODE (op) == LSHIFTRT >>> +#if defined(POINTERS_EXTEND_UNSIGNED) >>> + /* we skip this optimisation if pointers naturally extend signed */ >>> + && POINTERS_EXTEND_UNSIGNED >>> +#endif >>> && CONST_INT_P (XEXP (op, 1)) >>> && XEXP (op, 1) != const0_rtx) >>> return simplify_gen_unary (ZERO_EXTEND, mode, op, GET_MODE (op)); >> >> Is it just me or does this miss a || mode != Pmode || GET_MODE (op) != >> ptr_mode >> check? Note the comment says exactly the opposite as the transform... >> >> I’m not even sure why this simplification is correct in the first place?! > > I hope you are not confusing my use of POINTERS_EXTEND_UNSIGNED as a proxy > for the property that defines whether sub width operations sign-extend to the > full width of the register vs zero extend. Are you taking about our added > comment?
Read it as: riscv.h #define REGISTERS_EXTEND_UNSIGNED false +#if defined(REGISTERS_EXTEND_UNSIGNED) + /* we skip this optimisation if registers naturally extend signed */ + && REGISTERS_EXTEND_UNSIGNED +#endif The comment is the inverse because RISC-V registers naturally extend signed and REGISTERS_EXTEND_UNSIGNED is false. GCC defines unsigned false for RISC-V promotions.
