On Mon, Jun 10, 2019 at 9:14 PM Neil Schellenberger <[email protected]> wrote: > > When upgrading our build container from 1.11.10 to 1.12.5, some of our tests > failed in a surprising way. I've boiled down the issue to this short example > code fragment. > > package main > > // #include <string.h> > // struct S { unsigned char data[18]; }; > import "C" > import "unsafe" > > func main() { > var array [5]C.struct_S > for i := range array { > C.memset(unsafe.Pointer(&array[i].data[0]), 0xff, C.sizeof_struct_S) > } > for i := range array { > for j := range array[i].data { > if array[i].data[j] != C.uchar(0xff) { > panic("oops") > } > } > } > } > > Under 1.11 this runs to completion; under 1.12 it panics. > > Debugging and instrumenting shows that the problem is the first argument to > memset is not what one would expect when run under 1.12. (The address of a > heap temporary copy rather than the original perhaps?) > > Strangely, either parenthesising the argument expression to the address > operand ("&(array[i].data[0])") or removing the final index expression > ("&array[i].data") "fixes" the problem. > > I've spelunked through the AST (identical under both); the liveness and > escape analysis (different); the parse tree (different); and the go asm > (different). Frankly, my gc-fu is very weak and I don't really know what I'm > looking for. > > I assume that the root cause is that we were violating some sort of liveness > or escape policy, but I can't figure out what it would be (or why minor > syntactic changes "fix" things). I'd be quite curious to know. > > The workaround in the real code is simple, so this isn't a huge issue, just a > a bit of a mystery.
It's a bug. Would you mind opening an issue with this test case at https://golang.org/issue? Thanks. Ian -- 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 [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAOyqgcVK6wibdQTdssctnS9_N2WJCUqrboR3GaeiWxsM0Z7_WQ%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
