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Controlling Specific Optimizer Features on HP-UX
Most of the time, specifying optimization level 1, 2, 3, or 4 should
provide you with the control over the optimizer that you need.
Additional parameters are provided when you require a finer level of
control.
At each level, you can turn on and off specific optimizations using the
+O[no]optimization option. The optimization parameter is the name of a
specific optimization technique. The optional prefix [no] disables the
specified optimization.
Table 4-4 describes the optimizations that can be turned on or off:
Table 4-4. Advanced Optimization Options
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| | | |
| Optimization | What It Does | Level of |
| Option | | Opt |
| | | |
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| | | |
| +O[no]initcheck | The initialization checking feature of the optimizer | 2, 3, 4 |
| | has three possible states: on, off, or unspecified. | |
| | When on (+Oinitcheck), the optimizer initializes to | |
| | zero any local, scalar, non-static variables that are | |
| | uninitialized with respect to at least one path | |
| | leading to a use of the variable. | |
| | | |
| | When off (+Onoinitcheck), the optimizer issues | |
| | warning messages when it discovers definitely | |
| | uninitialized variables, but does not initialize | |
| | them. | |
| | | |
| | When unspecified, the optimizer initializes to zero | |
| | any local, scalar, non-static variables that are | |
| | definitely uninitialized with respect to all paths | |
| | leading to a use of the variable. | |
| | | |
| | Use +Oinitcheck to look for variables in a program | |
| | that may not be initialized. | |
| | | |
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| | | |
| +O[no]parmsoverlap | Optimize with the assumption that the actual | 2, 3, 4 |
| | arguments of function calls do [or do not] overlap in | |
| | memory. The +Onoparmsoverlap option replaces the | |
| | obsolete +Om1 option. | |
| | | |
| | The default is +Oparmsoverlap. | |
| | | |
| | Use +Onoparmsoverlap if C programs have been | |
| | literally translated from FORTRAN programs. | |
| | | |
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Table 4-4. Advanced Optimization Options (cont.)
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| | | |
| Optimization | What It Does | Level of |
| Option | | Opt |
| | | |
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| | | |
| +O[no]volatile | The +Ovolatile option implies that memory references | 1,2,3,4 |
| | to global variables cannot be removed during | |
| | optimization. | |
| | | |
| | The +Onovolatile option implies that all globals are | |
| | not of volatile class. This means that references to | |
| | global variables can be removed during optimization. | |
| | | |
| | | |
| | The +Ovolatile option replaces the obsolete +OV | |
| | option. | |
| | | |
| | The default is +Onovolatile. | |
| | | |
| | Use this toggle to control the "volatile" semantics | |
| | for all global variables. | |
| | | |
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| | | |
| +O[no]inline[=name1, | When +Oinline is specified without a name list, any | 3, 4 |
| name2, ...nameN] | function can be inlined. When specified with a name | |
| | list, the named functions are important candidates | |
| | for inlining. The +Onoinline option disables | |
| | inlining for all functions or a specific list. | |
| | | |
| | The default is +Oinline. | |
| | | |
| | Use this toggle when you need to precisely control | |
| | which functions are inlined. | |
| | | |
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Table 4-4. Advanced Optimization Options (cont.)
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| | | |
| Optimization | What It Does | Level of |
| Option | | Opt |
| | | |
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| | | |
| +O[no]libcalls | Expand [or do not expand] the following library calls | 0,1,2,3,4|
| | inline: strcpy(), sqrt(), fabs(), and alloca(). | |
| | Also use [or do not use] millicode instead of certain | |
| | math library calls. The +Olibcalls option replaces | |
| | the obsolete -J option. | |
| | | |
| | The default is +Onolibcalls. | |
| | | |
| | Use +Olibcalls to improve the performance of selected | |
| | library routines only when you are not performing | |
| | error checking for these routines. | |
| | | |
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| | | |
| +O[no]moveflops | Allows [or disallows] moving conditional floating | 2, 3, 4 |
| | point instructions out of loops. The +Onomoveflops | |
| | option replaces the obsolete +OE option. The | |
| | behavior of floating-point exception handling may be | |
| | altered by this option. | |
| | | |
| | The default is +Omoveflops. | |
| | | |
| | Use +Onomoveflops if floating-point traps are enabled | |
| | and you do not want the behavior of floating-point | |
| | exceptions to be altered by the relocation of | |
| | floating-point instructions. | |
| | | |
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Table 4-4. Advanced Optimization Options (cont.)
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| | | |
| Optimization | What It Does | Level of |
| Option | | Opt |
| | | |
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| | | |
| +O[no]pipeline | Enables [or disables] software pipelining. The | 2, 3, 4 |
| | +Onopipeline option replaces the obsolete +Os option. | |
| | | |
| | | |
| | The default is +Opipeline. | |
| | | |
| | Use +Onopipeline to conserve code space. | |
| | | |
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| | | |
| +O[no]entrysched | Optimizes instruction scheduling on a procedure's | 1,2,3,4 |
| | entry and exit sequences. | |
| | | |
| | The default is +Onoentrysched. | |
| | | |
| | This option can change the behavior of programs that | |
| | perform exception-handling or that handle | |
| | asynchronous interrupts. | |
| | | |
| | The behavior of setjmp and longjmp is not effected. | |
| | | |
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| | | |
| +O[no]regionsched | Apply aggressive scheduling techniques to move | 2, 3, 4 |
| | instructions across branches. This toggle is | |
| | incompatible with the linker -z option. If used with | |
| | -z, it may cause a SIGSEGV error at run-time. | |
| | | |
| | The default is +Onoregionsched. | |
| | | |
| | Use +Oregionsched to improve application run-time | |
| | speed. | |
| | | |
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| | | |
| +O[no]signedpointers | Perform [or do not perform] optimizations related to | 0,1,2,3,4|
| | treating pointers as signed quantities. Applications | |
| | that allocate shared memory and that compare a | |
| | pointer to shared memory with a pointer to private | |
| | memory may run incorrectly if this optimization is | |
| | enabled. | |
| | | |
| | The default is +Onosignedpointers. | |
| | | |
| | Use +Osignedpointers to improve application run-time | |
| | speed. | |
| | | |
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Table 4-4. Advanced Optimization Options (cont.)
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| | | |
| Optimization | What It Does | Level of |
| Option | | Opt |
| | | |
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| | | |
| +O[no]sideeffects= | Assume that subprograms specified in the name list | 2, 3, 4 |
| [name1, name2,...,nameN] | might modify global variables. Therefore, when | |
| | +Osideeffects is enabled the optimizer limits global | |
| | variable optimization. | |
| | | |
| | The default is to assume that all subprograms have | |
| | side effects unless the optimizer can determine that | |
| | there are none. | |
| | | |
| | Use +Onosideeffects if you know that the named | |
| | functions do not modify global variables and you wish | |
| | to achieve the best possible performance. | |
| | | |
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| | | |
| +O[no]regreassoc | If disabled, this toggle turns off register | 2, 3, 4 |
| | reassociation. | |
| | | |
| | The default is +Oregreassoc. | |
| | | |
| | Use +Onoregreassoc to disable register reassociation | |
| | if this optimization hinders the optimized | |
| | application performance. | |
| | | |
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| | | |
| +O[no]fastaccess | Optimize for fast access to global data items. | 2, 3, 4 |
| | | |
| | The default is +Onofastaccess at optimization levels | |
| | 2 and 3, and +Ofastaccess at optimization level 4. | |
| | | |
| | Use +Ofastaccess to improve execution speed at the | |
| | expense of longer compile times. | |
| | | |
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| | | |
| +O[no]fltacc | Disable or enable optimizations that cause imprecise | 2, 3, 4 |
| | floating-point results. | |
| | | |
| | The default is is +Ofltacc. +Ofltacc disables | |
| | optimizations that cause imprecise floating-point | |
| | results. | |
| | | |
| | Use +Onofltacc to improve execution speed at the | |
| | expense of floating-point precision. Include it in | |
| | the list of options that +Oaggressive sets. | |
| | | |
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