cmd/compile: out of fixed registers on 386 when building x/arch/vendor/rsc.io/pdf

[minux]

# golang.org/x/arch/vendor/rsc.io/pdf
run compiler with -v for register allocation sites
../../gopath/src/golang.org/x/arch/vendor/rsc.io/pdf/page.go:250: internal compiler error: out of fixed registers

[brian-brazil]

We've also run into this with Prometheus on both 1.5.3 and 1.6.0, see prometheus/prometheus#1509

[beorn7]

In case it helps, the offending code for prometheus/prometheus#1509 is

var (
    c []byte
    offset uint16
    bitMask [][]byte
)
// ...
c[offset/8] &^= bitMask[1][offset%8]

Breaking the statement into two helps. Both of the below work:

bm := bitMask[1][offset%8]
c[offset/8] &^= bm

newByte := c[offset/8] &^ bitMask[1][offset%8]
c[offset/8] = newByte

[gopherbot]

CL https://golang.org/cl/24942 mentions this issue.

[randall77]

This should be fixed with the new SSA backend.
(Although I can't reproduce the original problem.)

[gopherbot]

CL https://golang.org/cl/33247 mentions this issue.