Accelerator Manual (G06.24+, H06.03+)

Accelerator Performance Issues

Accelerator Manual—527303-002

6-15

Minimizing Compatibility Traps

Detection Guideline

•

The Accelerator issues Note 7 for each SETE instruction it detects.

Note 7: SETE instructions appear in the following

procedure(s): '<proc name>' at offset(s): <address>

Examine each SETE instruction to see if it changes the RP field of the ENV

Suggested Changes

•

Compiler-generated SETE instructions rarely cause transitions into TNS code.

•

Eliminate SETE instructions in TAL CODE statements that change the RP field.

Example

•

This code fragment disables overflow traps. The program does not make a

transition into TNS code because the SETE instruction does not change the RP

value.

CODE (RDE; ANRI %577; SETE); ! No transition into TNS code

Minimizing Compatibility Traps

Compatibility trap handlers are invisible system exception handlers that adjust the

results of certain otherwise incompatible TNS operations.

In nearly all cases, compatibility trap handlers do not significantly affect the

performance of accelerated programs. Accelerated code normally generates

compatibility traps on certain operations. (TNS object code executed on a TNS/R

system encounters the same compatibility traps.) These traps are expected, and often

require significant program revisions to be removed. Try to reduce the number of

compatibility traps for only the most performance-critical programs.

Odd-Byte References Traps

Odd-byte references to 2-byte words result in compatibility traps on TNS/R systems.

Odd-byte reference are made when the least significant bit (bit 31) of an extended

pointer is set and the pointer is used to access a variable or field larger than a byte.

TNS systems ignore bit 31 of an extended pointer and fetch data starting from the byte

prior to the addressed byte to ensure that the address is on a word boundary. On

TNS/R systems, a compatibility trap simulates this behavior, but only for 2-byte items.

The compatibility trap for this instruction simulation is about 90 times more expensive

than the well-aligned operation.

In TAL, data types stored as 2-byte words include INT and UNSIGNED(16). In C, data

types stored as 2-byte words include int, short, unsigned int, and unsigned short.