> 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. 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)
