cmd/compile: constants in -S output for arm64 have extra high bits set

[randall77]

package main

func f(x *int8) bool {
	return *x <= 43
}

Compiling this program with -S for arm64 has the line:

CMPW	$184683593771, R1

It should be

CMPW	$43, R1

Note that 184683593771 = 43<<32 + 43.
Go 1.16 is ok, this started in Go 1.17. Bisect points to https://go-review.googlesource.com/c/go/+/289649 . It looks maybe intentional? Not sure, but definitely weird.

The actual machine code looks correct. Builders are passing, and go tool objdump shows the right disassembly. It's just the -S output that appears broken.

@cherrymui @erifan

[erifan]

This problem has existed for a long time, but CL 289649 exposed it. The root cause is that we modified this constant operand in progedit by copying the lower 32 bits to the upper 32 bits.

We modify this constant just to determine whether it is a C_BITCON class and to encode it as a C_BITCON. So maybe we can do this modification (copy the lower 32 bits to the upper 32 bits) just before these two operations (con32class and bitconEncode ), and assign it to a temporary variable, so that the print result will not be modified.

Thanks for reporting this issue, I'll send a fix.

[gopherbot]

Change https://go.dev/cl/414374 mentions this issue: cmd/interna/obj/arm64: fix BITCON constant printing error