[PATCH] Global merge on constants

[Bill Wendling]

On Mar 14, 2013, at 4:53 PM, Quentin Colombet <qcolombet at apple.com> wrote:

> Thanks Bill, I appreciate even if it’s "a bit self-serving" :).
> 
> On Mar 14, 2013, at 4:24 PM, Bill Wendling <wendling at apple.com> wrote:
> 
>> On Mar 14, 2013, at 4:02 PM, Quentin Colombet <qcolombet at apple.com> wrote:
>> 
>>> Hi,
>>> 
>>> I’ve updated the patch for Global merge constants so that it implements the last proposal.
>>> 
>>> Thanks for your reviews,
>>> 
>> It's a bit self-serving for me to review this, given that I suggested it, but here we go. :-) Overall, this looks good. I would be interested to know how this affects compile time --- specifically at '-O0'. I ask because of looping over the uses of a global in a program can take a while for something that's used a lot. Could you run the test-suite to see if there are any significant slowdowns?
> Will do. However, this pass is obviously not run at O0.
> 
Ah! yes. I had a brain crash. :)

>> 
>> There are a few nits:
>> 
>> +/// Check whether the uses of the given value can be replaced by GEP.
>> 
>> Should this start with a '\brief’?
> Good question…
> doxygen can interpret the first line of such comments as a brief, but it depends on how it is configured so I don’t know.
> Anyway, it does not hurt to add one in that case :).
> 
>> +/// This function also checks transitively the uses made after a bitcast of the
>> +/// given value.
>> +/// Currently we consider that this is not possible when at least one of the
>> +/// use is one of these kind:
>> +/// - Intrinsic
>> +/// - Landing pad
>> +/// - Inline asm
>> +/// \pre Val is a GlobalVariable or a bitcast of a GlobalVariable or a bitcast
>> +///      of a bitcast of a GlobalVariable and so on.
>> +static bool areUsesAcceptGEP(const Value *Val) {
>> 
>> Do you mean 'areUsesExceptGEP’?
> No, I want to check if all the uses can have a GEP as one of their operand, i.e., it won’t break the IR if we turn one of their operand into a GEP.

Ah! okay. That makes sense.

> Maybe doUsesAcceptGEP is more correct?
> If you have something else in mind, I’m sure it will be better!
> 
How about 'canBeReplacedByGEP'?

>> +    if (CI && isa<const InlineAsm>(CI->getCalledValue()))
>> +      return false;
>> +    // Check transitively uses done after a bitcasts
>> +    const BitCastInst *BI = dyn_cast<const BitCastInst>(*UseIt);
>> +    // FIXME: Is it possible to have a cycle with bitcasts uses, i.e.,
>> +    // value1 bitcasted to value2 and value2 bitcasted to value1 without
>> +    // anything in between?
>> 
>> I don't think it's possible with SSA. I'm assuming you mean something like this?
> That’s what I think too: because of SSA and because you cannot change the address of a global variable.
> I can remove this comment then!
> 
> For the record, I had in mind something like this (which is syntactically wrong):
> @g = i32*
> @h = float*
> 
> define @foo() {
>   @h = bitcast i32* @g to float*
> }
> 
> define @bar() {
>   @g = bitcast float* @h to i32*
> }
> 
Yeah. I don't think you can do that. :)

-bw