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Re: basic asm and memory clobbers


On 11/20/2015 3:14 AM, Andrew Haley wrote:
On 20/11/15 10:37, David Wohlferd wrote:
The intent for 24414 is to change basic asm such that it will become
(quoting jeff) "an opaque blob that read/write/clobber any register or
memory location."  Such being the case, "memory" is not sufficient:

#define CLOBBERALL "eax", "ebx", "ecx", "edx", "r8", "r9", "r10", "r11",
"r12", "r13", "r14", "r15", "edi", "esi", "ebp", "cc", "memory"
Hmm.  I would not be at all surprised to see this cause reload
failures.  You certainly shouldn't clobber the frame pointer on
any machine which needs one.

If I don't clobber ebp, gcc just uses it:

        movl    $1000, %ebp
.L2:
        #
        subl    $1, %ebp
        jne     .L2

The original purpose of this code was to attempt to show that this kind of "clobbering everything" behavior (the proposed new behavior for basic asm) could have non-trivial impact on existing routines. While I've been told that changing the existing "clobber nothing" approach to this kind of "clobber everything" is "less intrusive than you might think," I'm struggling to believe it. It seems to me that one asm("nop") thrown into a driver routine to fix a timing problem could end up making a real mess.

But actually we're kind of past that. When Jeff, Segher, (other) Andrew and Richard all say "this is how it's going to work," it's time for me to set aside my reservations and move on.

So now I'm just trying my best to make sure that if it *is* an issue, people have a viable solution readily available. And to make sure it's all correctly doc'ed (which is what started this whole mess).

dw


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