simulation; vliw targets; writeback tracking bitstring overflow

simulation; vliw targets; writeback tracking bitstring overflow

[Frank Ch. Eigler]

Hi -

For an under-development cgen-based simulator port, we have a VLIW-type
machine that uses the "parallel-write?" method of concurrency modelling.
That means that during the semantic stage, a parexec struct is filled up
with write-queued operand values, and a bitmask field ("written") is
tweaked to indicate that a given field in the struct was in fact enqueued.

The problem is that this indicator bitfield is hard-coded as an "unsigned
int", meaning that it can be used to track just 32 operands.  My port of
interest manages to overflow this humble limit, and uses bit numbers on
the order 35 .. 37.  That obviously won't fit, and leads to failures.
(Actually, only the compiler warnings "left shift amount >= width" gave
away that something suspicious was going on!)

Anyway, this needs to get fixed.  A couple of alternatives are:

- widening the bitfield to "unsigned long long"
- adopting a separate per-field indicator variable
- (since I'm interested in sid -> C++) adopting a std::bitset<N> type

I'm tempted to do #1 for now, though #2 sounds straightforward too.
Opinions?

- FChE
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Re: simulation; vliw targets; writeback tracking bitstring overflow

[Ben Elliston]

   Anyway, this needs to get fixed.  A couple of alternatives are:

   - widening the bitfield to "unsigned long long"
   - adopting a separate per-field indicator variable
   - (since I'm interested in sid -> C++) adopting a std::bitset<N> type

What are the runtime memory requirements for moving to unsigned long long?

Ben

Re: simulation; vliw targets; writeback tracking bitstring overflow

[Frank Ch. Eigler]

Hi -

On Mon, Jan 29, 2001 at 01:25:40PM +1000, Ben Elliston wrote:
:    Anyway, this needs to get fixed.  A couple of alternatives are:
: 
:    - widening the bitfield to "unsigned long long"
:    - adopting a separate per-field indicator variable
:    - (since I'm interested in sid -> C++) adopting a std::bitset<N> type
: 
: What are the runtime memory requirements for moving to unsigned long long?

Effectively immeasurably small: an extra 4 bytes in an scache struct
and semantic function stack frame.  It's not really a big issue either
way.

- FChE
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