3.17.14 DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
- Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in libgcc.a will be used to perform floating-point
operations. Unless they are replaced by routines that emulate the
floating-point operations, or compiled in such a way as to call such
emulations routines, these routines will issue floating-point
operations. If you are compiling for an Alpha without floating-point
operations, you must ensure that the library is built so as not to call
Note that Alpha implementations without floating-point operations are
required to have floating-point registers.
- Generate code that uses (does not use) the floating-point register set.
-mno-fp-regs implies -msoft-float. If the floating-point
register set is not used, floating point operands are passed in integer
registers as if they were integers and floating-point results are passed
$0 instead of
$f0. This is a non-standard calling sequence,
so any function with a floating-point argument or return value called by code
compiled with -mno-fp-regs must also be compiled with that
A typical use of this option is building a kernel that does not use,
and hence need not save and restore, any floating-point registers.
- The Alpha architecture implements floating-point hardware optimized for
maximum performance. It is mostly compliant with the IEEE floating
point standard. However, for full compliance, software assistance is
required. This option generates code fully IEEE compliant code
except that the inexact-flag is not maintained (see below).
If this option is turned on, the preprocessor macro
defined during compilation. The resulting code is less efficient but is
able to correctly support denormalized numbers and exceptional IEEE
values such as not-a-number and plus/minus infinity. Other Alpha
compilers call this option -ieee_with_no_inexact.
- This is like -mieee except the generated code also maintains
the IEEE inexact-flag. Turning on this option causes the
generated code to implement fully-compliant IEEE math. In addition to
_IEEE_FP_EXACT is defined as a preprocessor
macro. On some Alpha implementations the resulting code may execute
significantly slower than the code generated by default. Since there is
very little code that depends on the inexact-flag, you should
normally not specify this option. Other Alpha compilers call this
- This option controls what floating-point related traps are enabled.
Other Alpha compilers call this option -fptm trap-mode.
The trap mode can be set to one of four values:
- This is the default (normal) setting. The only traps that are enabled
are the ones that cannot be disabled in software (e.g., division by zero
- In addition to the traps enabled by n, underflow traps are enabled
- Like su, but the instructions are marked to be safe for software
completion (see Alpha architecture manual for details).
- Like su, but inexact traps are enabled as well.
- Selects the IEEE rounding mode. Other Alpha compilers call this option
-fprm rounding-mode. The rounding-mode can be one
- Normal IEEE rounding mode. Floating point numbers are rounded towards
the nearest machine number or towards the even machine number in case
of a tie.
- Round towards minus infinity.
- Chopped rounding mode. Floating point numbers are rounded towards zero.
- Dynamic rounding mode. A field in the floating point control register
(fpcr, see Alpha architecture reference manual) controls the
rounding mode in effect. The C library initializes this register for
rounding towards plus infinity. Thus, unless your program modifies the
fpcr, d corresponds to round towards plus infinity.
- In the Alpha architecture, floating point traps are imprecise. This
means without software assistance it is impossible to recover from a
floating trap and program execution normally needs to be terminated.
GCC can generate code that can assist operating system trap handlers
in determining the exact location that caused a floating point trap.
Depending on the requirements of an application, different levels of
precisions can be selected:
- Program precision. This option is the default and means a trap handler
can only identify which program caused a floating point exception.
- Function precision. The trap handler can determine the function that
caused a floating point exception.
- Instruction precision. The trap handler can determine the exact
instruction that caused a floating point exception.
Other Alpha compilers provide the equivalent options called
-scope_safe and -resumption_safe.
- This option marks the generated code as IEEE conformant. You must not
use this option unless you also specify -mtrap-precision=i and either
-mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect
is to emit the line .eflag 48 in the function prologue of the
generated assembly file. Under DEC Unix, this has the effect that
IEEE-conformant math library routines will be linked in.
- Normally GCC examines a 32- or 64-bit integer constant to
see if it can construct it from smaller constants in two or three
instructions. If it cannot, it will output the constant as a literal and
generate code to load it from the data segment at runtime.
Use this option to require GCC to construct all integer constants
using code, even if it takes more instructions (the maximum is six).
You would typically use this option to build a shared library dynamic
loader. Itself a shared library, it must relocate itself in memory
before it can find the variables and constants in its own data segment.
- Select whether to generate code to be assembled by the vendor-supplied
assembler (-malpha-as) or by the GNU assembler -mgas.
- Indicate whether GCC should generate code to use the optional BWX,
CIX, FIX and MAX instruction sets. The default is to use the instruction
sets supported by the CPU type specified via -mcpu= option or that
of the CPU on which GCC was built if none was specified.
- Generate code that uses (does not use) VAX F and G floating point
arithmetic instead of IEEE single and double precision.
- Older Alpha assemblers provided no way to generate symbol relocations
except via assembler macros. Use of these macros does not allow
optimal instruction scheduling. GNU binutils as of version 2.12
supports a new syntax that allows the compiler to explicitly mark
which relocations should apply to which instructions. This option
is mostly useful for debugging, as GCC detects the capabilities of
the assembler when it is built and sets the default accordingly.
- When -mexplicit-relocs is in effect, static data is
accessed via gp-relative relocations. When -msmall-data
is used, objects 8 bytes long or smaller are placed in a small data area
.sbss sections) and are accessed via
16-bit relocations off of the
$gp register. This limits the
size of the small data area to 64KB, but allows the variables to be
directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area
is limited to just below 2GB. Programs that require more than 2GB of
data must use
mmap to allocate the data in the
heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies
-msmall-data and -fPIC implies -mlarge-data.
- When -msmall-text is used, the compiler assumes that the
code of the entire program (or shared library) fits in 4MB, and is
thus reachable with a branch instruction. When -msmall-data
is used, the compiler can assume that all local symbols share the
$gp value, and thus reduce the number of instructions
required for a function call from 4 to 1.
The default is -mlarge-text.
- Set the instruction set and instruction scheduling parameters for
machine type cpu_type. You can specify either the EV
style name or the corresponding chip number. GCC supports scheduling
parameters for the EV4, EV5 and EV6 family of processors and will
choose the default values for the instruction set from the processor
you specify. If you do not specify a processor type, GCC will default
to the processor on which the compiler was built.
Supported values for cpu_type are
- Schedules as an EV4 and has no instruction set extensions.
- Schedules as an EV5 and has no instruction set extensions.
- Schedules as an EV5 and supports the BWX extension.
- Schedules as an EV5 and supports the BWX and MAX extensions.
- Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
- Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
- Set only the instruction scheduling parameters for machine type
cpu_type. The instruction set is not changed.
- Sets the latency the scheduler should assume for typical memory
references as seen by the application. This number is highly
dependent on the memory access patterns used by the application
and the size of the external cache on the machine.
Valid options for time are
- A decimal number representing clock cycles.
- The compiler contains estimates of the number of clock cycles for
“typical” EV4 & EV5 hardware for the Level 1, 2 & 3 caches
(also called Dcache, Scache, and Bcache), as well as to main memory.
Note that L3 is only valid for EV5.