[LLVMdev] Metadata for Argument, BasicBlock

[Dan Gohman]

On May 9, 2012, at 2:29 AM, Richard Osborne wrote:

> On 07/05/12 22:58, Dan Gohman wrote:
>> What kind of things might basic block metadata be used for?
>> 
>> Dan
> I'd be really keen for this to go in. In order to support worse case execution time analysis on compiled binaries we (XMOS) need a way to mark paths that should be excluded when checking timing constraints. A typical query is "Check the worse case execution time from A to B excluding paths which pass through location C is no more than x nanoseconds". Here C would be a location marked in the source code. A natural way to implement this would be to have the frontend attach metadata to the basic block containing C associating the block with the label "C". Trying to attach the information to an individual instruction is problematic since that specific instruction might be removed / hoisted. Other solutions I can think of might prevent optimizations from happening (which is something we want to avoid).

Attaching metadata to a block isn't completely free of problems either, because blocks
can be merged, duplicated, and so on.

A possible alternative approach would be to use a special inline asm, like

  asm volatile ("# my special anchor");

(written here as C syntax because it's easier in email)

as an anchor to mark a position in the program.  Today, LLVM treats this as reading
and writing arbitrary memory, which blocks memory optimizations, but this seems
to be over-conservative, since GCC does not (GCC requires the explicit "memory"
clobber to indicate memory use). If this were fixed, such an inline asm would be a
convenient way to mark a position in a program in a way that wouldn't be lightly
deleted or moved, and it would have a minimal impact on optimization.

As long as LLVM's target-independent optimizers don't start interpreting inline asm
strings, which seems a sane assumption.

Dan