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0 pointsjoosters12y ago0 comments

Not a MOV and an ADD?

0 comments

You can use that method (arm usually uses a "mov" and "movt" to load upper and lower half-words), but you would be using 2 instructions to accomplish the same thing that ldr with a literal pool does in one.

joostersOP12y ago

Not sure about a MOVT (my ARM assembler is only current to an ARM2...) but it's two (pipelined) instructions vs one instruction and a slow memory access. But then again, I'm thinking of a previous era with no instruction or data caches !

tjdetwiler12y ago

I speak mostly from an ARMv7 perspective (so pipelines, caches, prefetchers, etc are par for the course).

Your literal pool has a high degree of spacial locality WRT the instruction that references it. You can take measures to optimize this (see the ltorg directive) making I$ hits very likely.

In the end, the assembler is free to pick and choose which method is best for the target CPU and input code.

j / k navigate · click thread line to collapse

0 comments

tjdetwiler12y ago

joostersOP12y ago

tjdetwiler12y ago

I speak mostly from an ARMv7 perspective (so pipelines, caches, prefetchers, etc are par for the course).

Your literal pool has a high degree of spacial locality WRT the instruction that references it. You can take measures to optimize this (see the ltorg directive) making I$ hits very likely.

In the end, the assembler is free to pick and choose which method is best for the target CPU and input code.

j / k navigate · click thread line to collapse