[PATCH] PR/27733, synth_mult cache missing too much

[Roger Sayle]

Hi Paolo,

On Thu, 8 Jun 2006, Paolo Bonzini wrote:
> 2006-06-08  Paolo Bonzini  <bonzini@gnu.org>
>
>	* expmed.c (struct alg_hash_entry): Fix type of field T
>	to match synth_mult argument.
>	(NUM_ALG_HASH_ENTRIES): Make it bigger for 64-bit HOST_WIDE_INT.

Wow!  Superb catch!  My plan for 4.3 was to avoid the generation of
RTL in tree-ssa-loop-ivopts.c:multiply_by_cost.  We spend a lot of
time whilst deciding whether to hoist a multiplication out of a loop
choosing between low latency sequences that have the same "cost",
which is often easy to estimate based on bit-counts.  This will get
worse with "multiply-by-11".  However, it looks like you've found a
much more effective fix.  It also explains why 64-bit targets, like
the alpha and hppa64, are so dramatically slower in synth_mult
than x86.

This patch is OK for mainline, and open release branches after a
suitable delay, once your bootstrapping and regression testing
complete.  It might even be worth removing the #ifdef around the
NUM_ALG_HASH_ENTRIES definition and use 1031 for all targets.
We have much worse memory vs. compile-time tradeoffs in the compiler,
but I'll leave that argument to a follow-up posting should anyone
care to measure the cost/benefit on 32-bit platforms.

Chapeau and Kudos,

Roger
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