Andrew Pinski <andrew.pin...@caviumnetworks.com> writes:
> Hi,
> For some reason cse produced:
> (insn 27 43 28 2 (set (reg:SI 231 [+4 ])
> (plus:SI (reg:SI 229 [+4 ])
> (const_int 0 [0]))) t.c:24 10 {*addsi3}
> (nil))
>
> (insn 28 27 29 2 (set (reg:SI 212)
> (ltu:SI (reg:SI 231 [+4 ])
> (reg:SI 229 [+4 ]))) t.c:24 521 {*sltu_sisi}
> (nil))
>
> And then forwprop goes and does the following:
> In insn 28, replacing
> (ltu:SI (reg:SI 231 [+4 ])
> (reg:SI 229 [+4 ]))
> with (const_int 1 [0x1])
>
> This is due to simplify-rtx.c (simplify_relational_operation_1) Having
> the following optimization:
> /* (LTU/GEU (PLUS a C) C), where C is constant, can be simplified to
> (GEU/LTU a -C). Likewise for (LTU/GEU (PLUS a C) a). */
>
> But this is wrong when C is 0. Anyways I committed this patch as
> obvious to fix the issue in simplify-rtx.c but I have not looked into
> why CSE does simplify the plus.
Hmm, like you said originally in the PR, I think generating the invalid
PLUS is the bug here. I agree (as Richard B said) that an invalid PLUS
shouldn't cause us to generate wrong code, but an assert seems better
than a check. Eric, what do you think?
Richard
> PR rtl-opt/54524
> * simplify-rtx.c (simplify_relational_operation_1): Don't simplify
> (LTU/GEU (PLUS a 0) 0) into (GEU/LTU a 0) since they are not
> equivalent.
>
> Index: simplify-rtx.c
> ===================================================================
> --- simplify-rtx.c (revision 193110)
> +++ simplify-rtx.c (working copy)
> @@ -4546,7 +4546,9 @@ simplify_relational_operation_1 (enum rt
> && GET_CODE (op0) == PLUS
> && CONST_INT_P (XEXP (op0, 1))
> && (rtx_equal_p (op1, XEXP (op0, 0))
> - || rtx_equal_p (op1, XEXP (op0, 1))))
> + || rtx_equal_p (op1, XEXP (op0, 1)))
> + /* (LTU/GEU (PLUS a 0) 0) is not the same as (GEU/LTU a 0). */
> + && XEXP (op0, 1) != const0_rtx)
> {
> rtx new_cmp
> = simplify_gen_unary (NEG, cmp_mode, XEXP (op0, 1), cmp_mode);
