runtime: stray pointer read in cmd/compile/internal/ssa test

[rsc]

Trybot run for https://go-review.googlesource.com/c/go/+/87155/3 produced this on darwin/amd64:

# cmd/compile/internal/ssa
unexpected fault address 0xb01dfacedebac1e
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb01dfacedebac1e pc=0x100178a]

goroutine 1 [running]:
runtime.throw(0x12c9a15, 0x5)
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/runtime/panic.go:616 +0x81 fp=0xc4276ab828 sp=0xc4276ab808 pc=0x1028fa1
runtime.sigpanic()
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/runtime/signal_unix.go:389 +0x200 fp=0xc4276ab878 sp=0xc4276ab828 pc=0x103d2c0
runtime.interhash(0xc4276ab9f0, 0x93dbd612, 0xc4331190a0)
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/runtime/alg.go:146 +0x3a fp=0xc4276ab8d0 sp=0xc4276ab878 pc=0x100178a
runtime.mapaccess2(0x127ac40, 0xc4250456b0, 0xc4276ab9f0, 0xc43453f788, 0xc424e9f001)
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/runtime/hashmap.go:412 +0x73 fp=0xc4276ab918 sp=0xc4276ab8d0 pc=0x10072d3
main.(*Package).growSpan(...)
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/cmd/vet/shadow.go:97
main.(*Package).check(0xc420445ce0, 0xc42005d0c0, 0xc424e9f000, 0x55, 0x80, 0x0, 0xc425053000, 0x0)
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/cmd/vet/types.go:109 +0x5e6 fp=0xc4276abb28 sp=0xc4276ab918 pc=0x1231396
main.doPackage(0xc4200ec000, 0x55, 0x5e, 0x0, 0x1433240)
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/cmd/vet/main.go:431 +0x7f1 fp=0xc4276abdd8 sp=0xc4276abb28 pc=0x1224cf1
main.doPackageCfg(0x7fff5fbff980, 0x4f)
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/cmd/vet/main.go:345 +0x221 fp=0xc4276abe88 sp=0xc4276abdd8 pc=0x1223c31
main.main()
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/cmd/vet/main.go:243 +0x243 fp=0xc4276abf88 sp=0xc4276abe88 pc=0x1223063
runtime.main()
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/runtime/proc.go:198 +0x212 fp=0xc4276abfe0 sp=0xc4276abf88 pc=0x102a5e2
runtime.goexit()
	/var/folders/dx/k53rs1s93538b4x20g46cj_w0000gn/T/workdir/go/src/runtime/asm_amd64.s:2361 +0x1 fp=0xc4276abfe8 sp=0xc4276abfe0 pc=0x1053121
FAIL	cmd/compile/internal/ssa [build failed]
2018/01/10 08:22:22 Failed: exit status 2

https://storage.googleapis.com/go-build-log/91a88879/darwin-amd64-10_11_dd529910.log

Seems bad and I don't see how my CL could have caused it. Earlier patch sets did not behave like that.

[bradfitz]

Maybe related: #22988 (runtime/Darwin memory corruption?)

[ianlancetaylor]

cc @aclements

[ianlancetaylor]

Just so others don't have to look this up, the fault address 0xb01dfacedebac1e means that Darwin raised a SIGSEGV because of a malformed address: one for which the top 16 bits are not a copy of bit 47. So this certainly appears to be some form of memory corruption. The error occurs when trying to fetch the hash function to use for a map key type, on the line fn := t.alg.hash. The map key type is an interface type, and t is the type of the value passed as the map key. Either t or t.alg appears to be corrupt.

[aclements]

Closing as a dup of #22988.