Not surprising, but it also repros with the original github.com/golang/protobuf/proto repo.

The seemingly relevant SSA ops are:

v1291 = FlagEQ <flags>
v932 = LoadReg <*bool> v2089 : DX
v794 = SETEQmem <mem> v932 v1291 v789

These are generated by lower from:

v449 = Const8 <byte> [10] (c[byte], c[byte], c[byte], c[byte], c[byte], c[byte])
v791 = Eq8 <bool> v449 v449
v794 = Store <mem> {bool} v742 v791 v789

I'm guessing the rewrite rules just need reordering so that we instead lower into moving constant 1 to memory.

Adding a rule for

(SETEQmem [off] {sym} ptr (FlagEQ) mem) -> (MOVBstore [off] {sym} ptr (MOVLconst [1]) mem)

seems to fix the ICE. Not sure if that's the best solution. (A complete fix would at least handle all the other SETCC/FlagCC combinations.)

/cc @cherrymui

Seems a regression from CL https://go-review.googlesource.com/c/go/+/67950. Indeed the handling of constant flags is missing. Adding @mdempsky's rule looks right to me.

cc @TocarIP

all the other SETCC/FlagCC combinations.

There are already rules (around line 1487 of AMD64.rules) that handles non-mem version of SETCC and constant flags. Seems we should add the similar ones for SETCCmem.

Here's the best I've been able to minimize the test case so far:

package p

func f(a *bool, b bool) {
        if b {
                return
        }
        c := '\n'
        if b {
                c = ' '
        }
        *a = c == '\n'
}

Also, the repro happens even without -l=4. It just happens to have been discovered in the protobuf code using -l=4.

Change https://golang.org/cl/69990 mentions this issue: cmd/compile: fold constant comparisions into SETxxmem ops.