On Tue, 22 Jun 2021, Qing Zhao wrote: > > > > On Jun 22, 2021, at 9:00 AM, Richard Biener <rguent...@suse.de> wrote: > > > > On Tue, 22 Jun 2021, Qing Zhao wrote: > > > >> So, I am wondering why not still keep my current implementation on > >> assign different patterns for different types? > >> > >> This major issue with this design is the code size and runtime overhead, > >> but for debugging purpose, those are not that important, right? And we > >> can add some optimization later to improve the code size and runtime > >> overhead. > >> > >> Otherwise, if we only use one pattern for all the types in this initial > >> version, later we still might need change it. > >> > >> How do you think? > > > > No, let's not re-open that discussion. As said we can look to support > > multi-byte pattern if that has a chance to improve things but only > > as followup. > > I am fine with this. > > However, we need to decide whether we will use one-byte repeatable pattern, > or multiple-byte repeatable pattern now, > Since the implementation will be different. If using one-byte, the > implementation will be the simplest, we can use memset for all > VLA, non-vla, zero-init, or pattern-init consistently. > > However, if we choose multiple-byte pattern, then the implementation will be > different, we cannot use memset for pattern-init, and > The implemenation for VLA pattern-init also is different.
As said, we can do this as followup. For now get the easiest thing working - one-byte patterns via memset. There's enough bits in the patch that will likely need followup fixes (the .DEFERED_INIT stuff), actual code gneration of the init is separate enough we can deal with it later. Also IMHO not all targets necessarily need to behave the same there. Richard. > Qing > > > > Thanks, > > Richard. > > > >> Qing > >> > >> On Jun 22, 2021, at 3:59 AM, Richard Biener > >> <rguent...@suse.de<mailto:rguent...@suse.de>> wrote: > >> > >> On Tue, 22 Jun 2021, Richard Sandiford wrote: > >> > >> Kees Cook <keesc...@chromium.org<mailto:keesc...@chromium.org>> writes: > >> On Mon, Jun 21, 2021 at 03:39:45PM +0000, Qing Zhao wrote: > >> So, if “pattern value” is “0xFFFFFFFFFFFFFFFF”, then it’s a valid > >> canonical virtual memory address. However, for most OS, > >> “0xFFFFFFFFFFFFFFFF” should be not in user space. > >> > >> My question is, is “0xFFFFFFFFFFFFFFFFF” good for pointer? Or > >> “0xAAAAAAAAAAAAAAAA” better? > >> > >> I think 0xFF repeating is fine for this version. Everything else is a > >> "nice to have" for the pattern-init, IMO. :) > >> > >> Sorry to be awkward, but 0xFF seems worse than 0xAA to me. > >> > >> For integer types, all values are valid representations, and we're > >> relying on the pattern being “obviously” wrong in context. 0xAAAA… > >> is unlikely to be a correct integer but 0xFFFF… would instead be a > >> “nice” -1. It would be difficult to tell in a debugger that a -1 > >> came from pattern init rather than a deliberate choice. > >> > >> I agree that, all other things being equal, it would be nice to use NaNs > >> for floats. But relying on wrong numerical values for floats doesn't > >> seem worse than doing that for integers. > >> > >> 0xAA… for float is (if I've got this right) -3.0316488252093987e-13, > >> which admittedly doesn't stand out as wrong. But I'm not sure we > >> should sacrifice integer debugging for float debugging here. > >> > >> We can always expose the actual value as --param. Now, I think > >> we'd need a two-byte pattern to reliably produce NaNs anyway, > >> so with floats taken out of the picture the focus should be on > >> pointers where IMHO val & 1 and val & 15 would be nice to have. > >> So sth like 0xf7 would work for those. With a two-byte pattern > >> we could use 0xffef or 0x7fef. > >> > >> Anyway, it's probably down to priorities of the project involved > >> (debugging FP stuff or integer stuff). > >> > >> Richard. > >> > >> > > > > -- > > Richard Biener <rguent...@suse.de> > > SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg, > > Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg) > > -- Richard Biener <rguent...@suse.de> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
