RFR(M): 8166562: C2: Suppress relocations in scratch emit.

[Vladimir Kozlov]

Looks good. I thought about using new type of CodeBlobType but it may need more changes then in your.

Thanks,
Vladimir

On 9/23/16 3:58 AM, Lindenmaier, Goetz wrote:
> Hi,
>
> Please review this nice and small improvement to scratch emit. It simplifies
>
> The s390 port considerably, but is completely independent. I introduced
>
> usage of the feature on ppc.   I please need a sponsor.
>
> http://cr.openjdk.java.net/~goetz/wr16/8166562-scratch_emit/webrev.01/
>
> The C2 compiler needs to know how much space the assembly emitted for a MachNode requires. For many nodes, this is statically specified. Some nodes don't have fixed sizes, as the code emitted depends
> on flags or even runtime values. To determine the sizes of these, C2 does a scratch emit, i.e., it emits the assembly for the MachNode to a dedicated code buffer and remembers the space needed. In the
> debug build, this is done on each emit also for nodes with fixed size to verify the fixed size.
>
> The scratch emit buffer does not support relocations. Therefore any code needing relocations must check for scratch emit and skip the relocations if so.
>
> The s390x architecture offers a lot of instructions with pc-relative addressing. We use these to access constants in the constant section of the code buffer. As this section can be resized, these
> offsets must be able to be relocated. Instead of coding the check whether a scratch emit is happening into all the MachNodes, we mark the scratch emit buffers as such and just skip the relocation in
> these buffers. This simplifies usage of relocations in a lot of nodes and macroAssembler routines.
>
> Best regards,
>
>    Goetz.
>