Yes, as Keith said, the 16-byte alignment requirement of sp is for the size of the entire stack, and there is no such requirement for a single variable on the stack. We can access a single variable through sp+/-offset. The code has already dealt with the overall alignment of sp. I tried to remove the alignment requirement for a single variable on linux/arm64, and all the tests in all.bash passed. So I think this alignment requirement for a single variable is unnecessary on linux/arm64. In addition, I also checked the result generated by gcc. In the case of -O0, the alignment of a single variable on the stack is consistent with its own size.
在2020年11月5日星期四 UTC+8 上午8:21:34<Keith Randall> 写道： > Ah, arm64. > That code jesper points to looks like the culprit. > The whole stack frame alignment should be handled separately from the > alignment of individual variables, so the total alignment needed shouldn't > matter. > > Not sure why we do that. A small archaeology dive indicates this code > dates from when the compiler was written in C. No indication of why it was > added. > I've opened an issue to look at this. > https://github.com/golang/go/issues/42386 > On Wednesday, November 4, 2020 at 5:51:03 AM UTC-8 jesper.lou...@gmail.com > wrote: > >> On Wed, Nov 4, 2020 at 9:36 AM eric...@arm.com <eric...@arm.com> wrote: >> >>> I got it, the code is here >>> https://github.com/golang/go/blob/633f9e206045176a12c301eb2c249c1c1d9a5d2e/src/cmd/compile/internal/gc/pgen.go#L186 >>> , >>> I don't know why the slot is required to be aligned at least 8 bytes on >>> these architectures. I will try to see if this requirement can be removed >>> on linux/arm64, >>> and to determine the slot size based on the actual size of the variable, >>> just like linux/amd64 does. >>> >>> >> If my memory serves (and it is spotty, you should definitely look it up) >> arm64 requires sp to be 16-byte aligned (or two times the word size to be >> precise), and this is a hardware requirement if accessing memory. >> >> There might be other reasons to want aligned access pertaining to >> atomicity guarantees and memory models. If the garbage collector assumes >> certain things about memory access when it is doing its unspeakable things >> to the stack, you may want it to have alignment to ensure certain >> operations behave nicely. Tread carefully, my friend. >> >> >> -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/6a52dee6-bf89-4365-ad95-96af119fc261n%40googlegroups.com.
