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Re: Question on param MAX_PENDING_LIST_LENGTH in sched-deps


On 11/04/14 20:29, Bin.Cheng wrote:
Hi,
The parameter MAX_PENDING_LIST_LENGTH is set to 32 by default.  It
seems to me the length of pending list can't be larger than 32.  But
in sched-deps.c, below code is used:
       /* Pending lists can't get larger with a readonly context.  */
       if (!deps->readonly
           && ((deps->pending_read_list_length + deps->pending_write_list_length)
               > MAX_PENDING_LIST_LENGTH))

Since we compares it using ">", the list can have 33 instructions at
most, which is inconsistent with the parameter name.

Well, it's some kind of nit picking.
Yea, seems like a bit of a nit. Might be worth fixing as someone might size an array based on the param's value only to find out it can have an extra element.


Another question.  This parameter is introduced to prevent GCC from
running for too long time.  I am not clear if the running time is a
quadratic function of pending list, or a quadratic function of
dependency nodes?  If it's the latter, could we use the number of
dependency nodes as the parameter directly?  Because in some cases
like memory initialization, there are more than 32 store instructions
in flow, but the dependency are actually very simple.
See:

https://gcc.gnu.org/ml/gcc-patches/2001-07/msg01668.html

I don't recall what part of the algorithm was quadratic, but it should be too hard to find out given the thread references the target & the testcase.

jeff


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