With some changes I was working on, I produced a situation where one of
the x86 peepholes made an incorrect transformation. In the prologue
expansion code, we have
/* When using red zone we may start register saving before allocating
the stack frame saving one cycle of the prologue. However, avoid
doing this if we have to probe the stack; at least on x86_64 the
stack probe can turn into a call that clobbers a red zone location.
So, we can in some situations produce something like:
mov %ebx, -8(%rsp)
mov %edi, -16(%rsp)
subl $16, %rsp
which is fine if using a redzone. The problem is that there are
peepholes which convert sp subtractions into pushes, clobbering the
saved registers.
I've made these peepholes conditional on !x86_using_red_zone.
Bootstrapped and tested on x86_64-linux, ok (now or later)?