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. 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)
