On Thu, 7 Jan 2021 at 13:56, Richard Earnshaw <richard.earns...@foss.arm.com> wrote: > > On 07/01/2021 00:59, Daniel Engel wrote: > > --snip-- > > > > On Wed, Jan 6, 2021, at 9:05 AM, Richard Earnshaw wrote: > > > >> > >> Thanks for working on this, Daniel. > >> > >> This is clearly stage1 material, so we've got time for a couple of > >> iterations to sort things out. > > > > I appreciate your feedback. I had been hoping that with no regressions > > this might still be eligible for stage2. Christophe never indicated > > either way. but the fact that he was looking at it seemed positive. > > I thought I would be a couple of weeks faster with this last > > iteration, but holidays got in the way. > > GCC doesn't have a stage 2 any more (historical wart). We were in > (late) stage3 when this was first posted, and because of the significant > impact this might have on not just CM0 but other targets as well, I > don't think it's something we should try to squeeze in at the last > minute. We're now in stage 4, so that is doubly the case. > > Christophe is a very valuable member of our community, but he's not a > port maintainer and thus cannot really rule on what can go into the > tools, or when. > > > > > I actually think your comments below could all be addressable within a > > couple of days. But, I'm not accounting for the review process. > > > >> Firstly, the patch is very large, but contains a large number of > >> distinct changes, so it would really benefit from being broken down into > >> a number of distinct patches. This will make reviewing the individual > >> changes much more straight-forward. > > > > I have no context for "large" or "small" with respect to gcc. This > > patch comprises about 30% of a previously-monolithic library that's > > been shipping since ~2016 (the rest is libm material). Other than > > (1) the aforementioned change to div0(), (2) a nascent adaptation > > for __truncdfsf2() (not enabled), and (3) the gratuitous addition of > > the bitwise functions, the library remains pretty much as it was > > originally released. > > Large, like many other terms is relative. For assembler file changes, > which this is primarily, the overall size can be much smaller and still > be considered 'large'. > > > > > The driving force in the development of this library was small size, > > which of course was never possible with the softfp routines. It's not > > half-slow, either, for the limitations of the M0 architecture. And, > > it's IEEE compliant. But, that means that most of the functions are > > highly interconnected. So, some of it can be broken up as you outline > > below, but that last patch is still worth more than half of the total. > > Nevertheless, having the floating-point code separated out will make > reviewing more straight forward. I'll likely need to ask one of our FP > experts to have a specific look at that part and that will be easier if > it is disentangled from the other changes. > > > > > I also have ~70k lines of test vectors that seem mostly redundant, but > > not completely. I haven't decided what to do here. For example, I have > > coverage for __aeabi_u/ldivmod, while GCC does not. If I do anything > > with this code it will be in a separate thread. > > Publishing the test code, even if it isn't integrated into the GCC > testsuite would be useful. Perhaps someone else could then help with that. > > > > >> I'd suggest: > >> > >> 1) Some basic makefile cleanups to ease initial integration - in > >> particular where we have things like > >> > >> LIB1FUNCS += <long list of functions> > >> > >> that this be rewritten with one function per line (and sorted > >> alphabetically) - then we can see which functions are being changed in > >> subsequent patches. It makes the Makefile fragments longer, but the > >> improvement in clarity for makes this worthwhile. > > > > I know next to nothing about Makefiles, particularly ones as complex as > > GCC's. I was just trying to work with the existing style to avoid > > breaking something. However, I can certainly adopt this suggestion. > > > >> 2) The changes for the existing integer functions - preferably one > >> function per patch. > >> > >> 3) The new integer functions that you're adding > > > > These wouldn't be too hard to do, but what are the expectations for > > testing? A clean build of GCC takes about 6 hours in my VM, and > > regression testing takes about 4 hours per architecture. You would want > > a full regression report for each incremental patch? I have no idea how > > to target regression tests that apply to particular runtime functions > > without the risk of missing something. > > > > Most of this can be tested in a cross-compile environment using qemu as > a model. A cross build shouldn't take that long (especially if you > restrict the compiler to just C and C++ - other languages are > vanishingly unlikely to pick up errors in the parts of the compiler > you're changing). But build checks will be mostly sufficient for most > of the intermediate patches. > > >> 4) The floating-point support. > >> > >> Some more general observations: > >> > >> - where functions are already in lib1funcs.asm, please leave them there. > > > > I guess I have a different vision here. I have had a really hard time > > following all of the nested #ifdefs in lib1funcs, so I thought it would > > be helpful to begin breaking it up into logical units. > > Agreed, it's not easy. But the restructuring, if any, should be done > separately from other changes, not mixed up with them. > > > > > The functions removed were all functions for which I had THUMB1 > > sequences faster/smaller than lib1funcs: __clzsi2, __clzdi2, __ctzsi2, > > __ashrdi3, __lshrdi3, __ashldi3. In fact, the new THUMB1 of __clzsi2 is > > the same number of instructions as the previous ARM/THUMB2 version. > > > > You will find all of the previous ARM versions of these functions merged > > into the new files (with attribution) and the same preprocessor > > selection path. So no architecture variant should be any worse off than > > before this patch, and some beyond v6m should benefit. > > > > In the future, I think that my versions of __divsi3 and __divdi3 will > > prove faster/smaller than the existing THUMB2 versions. I know that my > > float routines are less than half the compiled size of THUMB2 versions > > in 'ieee754-sf.S'. However, I haven't profiled the exact performance > > differences so I have left all this work for future patches. (It's also > > quite likely that my version can be further-refined with a few judicious > > uses of THUMB2 alternatives.) > > > > My long-term vision would be use lib1funcs as an architectural wrapper > > distinct from the implementation code. > > > >> - lets avoid having the cm0 subdirectory - in particular we should do > >> this when there is existing code for other targets in the same source > >> files. It's OK to have any new files in the main 'arm' directory of the > >> source tree - just name the files appropriately if really needed. > > > > Fair point on the name. In v1 of this patch, all these files were all > > preprocessor-selected for v6m only. However, as I've stumbled through > > the finer points of integration, that line has blurred. Name aside, > > the subdirectory does still represent a standalone library. I think > > I've managed to add enough integration hooks that it works well in > > a libgcc context, but it still has a very distinct implementation style. > > > > I don't have a strong opinion on this, just preference. But, keeping > > the subdirectory with a neutral name will probably make maintenance > > easier in the short term. I would suggest "lib0" (since it caters to > > the lowest common denominator) or "eabi" (since that was the original > > target). There are precedents in other architectures (e.g. avr). > > The issue here is that the selection of code from the various > subdirectories is not consistent. In some cases we might be pulling in > a thumb1 implementation into a thumb2 environment, so having the code in > a directory that doesn't reflect this makes maintaining the code harder. > I don't mind too much if some new files are introduced and their names > reflect both their function and the architecture they support - eg > t1-di-shift.S would obviously contain code for di-mode shifts in thumb1. > > > > >> - let's avoid the CM0 prefix - this is 'thumb1' code, for want of a > >> better term, and that is used widely elsewhere in the compiler. So if > >> you really need a term just use THUMB1, or even T1. > > > > Maybe. The Cortex M0 includes a subset of THUMB2 instructions. Most > > of this is probably THUMB1 clean, but it wasn't a design requirement. > > It's particularly the Thumb1 issue, just more the name is for a specific > CPU which might cause confusion later. v6m would be preferable to that > if there really is a dependency on the instructions that are not in the > original Thumb1 ISA. > > > > > The CM0_FUNC_START exists so that I can specify subsections of ".text" > > for each function. This was a fairly fundamental design decision that > > allowed me to make a number of branch optimizations between functions. > > The other macros are just duplicates for naming symmetry. > > This is something we'll have to get to during the main review of the > code - we used to have support for PE-COFF object files. That might now > be obsolete, wince support is certainly deprecated - but we can't assume > that ELF is the only object format we'll ever have to support. > > > > > The existing FUNC_START macro inserts extra conflicting ".text" > > directives that would break the build. Of course, the prefix was > > arbitrary; I just took CM0 from the library name. But, there's nothing > > architecturally significant about this macro at all, so THUMB1 and T1 > > seems just about as wrong. Maybe define a FUNC_START_SECTION macro with > > two parameters? For example: > > > > FUNC_START_SECTION clzdi2 .text.sorted.libgcc.clz2.clzdi2 > > > > Instead of: > > > > .section .text.sorted.libgcc.clz2.clzdi2,"x" > > CM0_FUNC_START clzdi2 > > > >> - For the 64-bit shift functions, I expect the existing code to be > >> preferable whenever possible - I think it can even be tweaked to build > >> for thumb2 by inserting a suitable IT instruction. So your code should > >> only be used when > >> > >> #if !__ARM_ARCH_ISA_ARM && __ARM_ARCH_ISA_THUMB == 1 > > > > That is the definition of NOT_ISA_TARGET_32BIT, which I am respecting. > > (The name doesn't seem quite right for Cortex M0, since it does support > > some 32 bit instructions, but that's beside the point.) > > The terms Thumb1 and Thumb2 predate the arm-v8m architecture > specifications - even then the term Thumb1 was interpreted as "mostly > 16-bit instructions" and thumb2 as "a mix of 16- and 32-bit". Yes, the > 16/32-bit spilt has become more blurred and that will likely continue in > future since the 16-bit encoding space is pretty full. > > > > > The current lib1funcs ARM code path still exists, as described above. My > > THUMB1 implementations were 1 - 3 instructions shorter than the current > > versions, which is why I took the time to merge the files. > > > > Unfortunately, the Cortex M0 THUMB2 subset does not provide IT. I don't > > see an advantage to eliminating the branch unless these functions were > > written with cryptographic side channel attacks in mind. > > On high performance cores branches are predicted - if the branch is > predictable then the common path will be taken and the unneeded > instructions will never be used. But library functions like this tend > to have very unpredictable values used for calling them, so it's much > less likely that the hardware will predict the right path - at this > point conditional instructions tend to win (especially if there aren't > very many of them) because the cost (on average) of not executing the > unneeded instructions is much lower than the cost (on average) of > unwinding the processor state to execute the other arm of the > conditional branch. > > > > >> - most, if not all, of your LSYM symbols should not be needed after > >> assembly, so should start with a captial 'L' (and no leading > >> underscores), the assembler will then automatically discard any that are > >> not needed for relocations. > > > > You don't want debugging symbols for libgcc internals :) ? I sort of > > understand that, but those symbols have been useful to me in the past. > > The "." by itself seems to keep visibility local, so the extra symbols > > won't cause linker issuess. Would you object to a macro variant (e.g. > > LLSYM) that prepends the "L" but is easier to disable? > > It is a matter of taste, but I really prefer the local symbols to > disappear entirely once the file is compiled - it makes things like > backtrace gdb show the proper call heirarchy. > > > > >> - you'll need to write suitable commit messages for each patch, which > >> also contain a suitable ChangeLog style entry. > > > > OK. > > > >> - finally, your popcount implementations have data in the code segment. > >> That's going to cause problems when we have compilation options such as > >> -mpure-code. > > > > I am just following the precedent of existing lib1funcs (e.g. __clz2si). > > If this matters, you'll need to point in the right direction for the > > fix. I'm not sure it does matter, since these functions are PIC anyway. > > That might be a bug in the clz implementations - Christophe: Any thoughts? >
Indeed that looks suspicious. I'm wondering why I saw no problem during testing. Is it possible that __clzsi2 is not covered by GCC's 'make check' ? > > > >> I strongly suggest that, rather than using gcc snapshots (I'm assuming > >> this based on the diff style and directory naming in your patches), you > >> switch to using a git tree, then you'll be able to use tools such as > >> rebasing and the git posting tools to send the patch series for > >> subsequent review. > > > > Your assumption is correct. I didn't think that I would have to get so > > deep into the gcc development process for this contribution. I used > > this library as a bare metal alternative for libgcc/libm in the product > > for years, so I thought it would just drop in. But, the libgcc compile > > mechanics have proved much more 'interesting'. I'm assuming this > > architecture was created years before the introduction of -ffunction- > > sections... > > > > I don't think I've time to write a history lesson, even if you wanted > it. Suffice to say, this does date back to the days of a.out format > object files (with 4 relocation types, STABS debugging, and one code, > one data and one bss section). > > >> > >> Richard. > >> > > > > Thanks again, > > Daniel > > > > R.