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[67.1.159.105]) by smtp.gmail.com with ESMTPSA id jc18-20020a17090325d200b001d9537cf238sm2096972plb.295.2024.02.02.14.48.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 02 Feb 2024 14:48:19 -0800 (PST) Message-ID: <3f212170-f555-4f78-aff9-2ebdb8d4cc78@rivosinc.com> Date: Fri, 2 Feb 2024 15:48:18 -0700 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] combine: Don't optimize SIGN_EXTEND of MEM on WORD_REGISTER_OPERATIONS targets [PR113010] Content-Language: en-US To: Jeff Law , Richard Biener Cc: gcc-patches@gcc.gnu.org References: <20240116221914.267015-1-gkm@rivosinc.com> <03b351a0-a5f4-48ab-bc67-765eabf5dbcd@rivosinc.com> From: Greg McGary In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-3.2 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 2/1/24 10:24 PM, Jeff Law wrote: > > On 2/1/24 18:24, Greg McGary wrote: > >> However, for a machine where (WORD_REGISTER_OPERATIONS && >> load_extend_op (inner_mode) == SIGN_EXTEND), the high part of a PSoM >> is  only known at runtime as 0s or 1s. That's the downstream bug. The >> fix for such machines is either (A) forbid static evaluation of the >> high part of a PSoM, or (B) forbid transforming (SIGN_EXTEND (MEM >> ...) ) into a PSoM. My patch does B. Perhaps you prefer A? The >> trouble with A is that in the zero-extend case, it is valid to >> statically evaluate as 0. It is only the sign-extend case that isn't >> known until runtime. By the time we have a PSoM, its upstream >> ancestor as sign- or zero-extend is already lost. >> >> Does that give you the understanding you desire, or are there deeper >> mysteries to probe? > It's a good start and I can see what you're trying to do -- and it may > in fact be correct -- the quick discussion with Palmer Tuesday and > your follow-up have helped a lot). > > But just to be sure, what's the incoming rtl at function entry? just > "debug_rtx (x)" should be sufficient. input: (sign_extend:DI (mem/c:SI (symbol_ref:DI ("minus_1") [flags 0x86] ) [1 minus_1+0 S4 A32])) result: (subreg:DI (mem/c:SI (symbol_ref:DI ("minus_1") [flags 0x86] ) [1 minus_1+0 S4 A32]) 0) Later, the high part of the PSoM statically evaluates to 0, the code to load and test is elided, and the incorrect alternative is emitted unconditionally. G