On 09/23/2017 04:57 PM, Eric Gallager wrote: > On Sat, Sep 23, 2017 at 12:34 PM, nick <xerofo...@gmail.com> wrote: >> >> >> On 2017-09-23 12:05 PM, Jeff Law wrote: >>> On 09/22/2017 08:25 PM, nick wrote: >>>> Greetings All, >>>> >>>> I am wondering if this is a warning worth looking into or is it just >>>> another false postive: >>>> >>>> /home/nick/gcc/gcc/combine.c:1316:8: warning: ‘prev’ may be used >>>> uninitialized in this function [-Wmaybe-uninitialized] >>>> if ((next = try_combine (insn, prev, NULL, NULL, >>>> >>>> Maybe it's just me being new to the project but is the code for that >>>> literally or also identical between this and the >>>> one where CC0 register is directly used. >>> Looks like a false positive to me. Without more details (host, target, >>> flags, cpp output) I can't be 100% sure why it triggers though. >>> >>> Flow sensitive warnings such as this are very sensitive to a variety of >>> low level target and IL details. >>> >>> >>> >>> Jeff >>> >> >> Jeff, >> >> Thanks specifically what low level details do you need. It's currently on a >> x86-64 bit ubuntu host. >> Was build with the stock gcc from the ubuntu 17.04 repos with W=2 for extra >> warnings. What do these >> lines do, anything outside of the normal setting marcos as they seem to me: >> >> && GET_CODE (PATTERN (insn)) == SET >> && reg_mentioned_p (cc0_rtx, SET_SRC (PATTERN (insn))) >> >> I am wondering what GET_CODE,SET_SRC, PATTERN marcos do plus the function >> called, reg_mentioned_P? >> If your able to just tell me where the functions are located or how do you >> enable ctags for all of >> gcc? That would just save me asking stupid questions. Is there a global >> setting like make ctags for >> doing this or you I have to do it manually. >> >> Thanks for the quick response, >> >> Nick > > I think I remember running into this warning before; it's because > stage 1 of bootstrap is built without optimization so the warnings > don't have as good information. It's certainly the case that at -O0, -O1 or -Os you'll tend to get more -Wuninitialized warnings.
Each of those throttle back jump threading which is vital to identifying and eliminating paths through the CFG that can never be executed. And a false positive -Wuninitialized warning is almost always on a path that can never be executed. jeff
