3.17.24 S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float-msoft-float- Use (do not use) the hardware floating-point instructions and registers
for floating-point operations. When -msoft-float is specified,
functions in libgcc.a will be used to perform floating-point
operations. When -mhard-float is specified, the compiler
generates IEEE floating-point instructions. This is the default.
-mbackchain-mno-backchain- Store (do not store) the address of the caller's frame as backchain pointer
into the callee's stack frame.
A backchain may be needed to allow debugging using tools that do not understand
DWARF-2 call frame information.
When -mno-packed-stack is in effect, the backchain pointer is stored
at the bottom of the stack frame; when -mpacked-stack is in effect,
the backchain is placed into the topmost word of the 96/160 byte register
save area.
In general, code compiled with -mbackchain is call-compatible with
code compiled with -mmo-backchain; however, use of the backchain
for debugging purposes usually requires that the whole binary is built with
-mbackchain. Note that the combination of -mbackchain and
-mpacked-stack generates code that is not ABI-compatible.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack- Use (do not use) the packed stack layout. When -mno-packed-stack is
specified, the compiler uses the all fields of the 96/160 byte register save
area only for their default purpose; unused fields still take up stack space.
When -mpacked-stack is specified, register save slots are densely
packed at the top of the register save area; unused space is reused for other
purposes, allowing for more efficient use of the available stack space.
However, when -mbackchain is also in effect, the topmost word of
the save area is always used to store the backchain, and the return address
register is always saved two words below the backchain.
As long as the stack frame backchain is not used, code generated with
-mpacked-stack is call-compatible with code generated with
-mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for
S/390 or zSeries generated code that uses the stack frame backchain at run
time, not just for debugging purposes. Such code is not call-compatible
with code compiled with -mpacked-stack. Also, note that the
combination of -mbackchain and -mpacked-stack generates code
that is not ABI-compatible.
The default is to not use the packed stack layout.
-msmall-exec-mno-small-exec- Generate (or do not generate) code using the
bras instruction
to do subroutine calls.
This only works reliably if the total executable size does not
exceed 64k. The default is to use the basr instruction instead,
which does not have this limitation.
-m64-m31- When -m31 is specified, generate code compliant to the
GNU/Linux for S/390 ABI. When -m64 is specified, generate
code compliant to the GNU/Linux for zSeries ABI. This allows GCC in
particular to generate 64-bit instructions. For the s390
targets, the default is -m31, while the s390x
targets default to -m64.
-mzarch-mesa- When -mzarch is specified, generate code using the
instructions available on z/Architecture.
When -mesa is specified, generate code using the
instructions available on ESA/390. Note that -mesa is
not possible with -m64.
When generating code compliant to the GNU/Linux for S/390 ABI,
the default is -mesa. When generating code compliant
to the GNU/Linux for zSeries ABI, the default is -mzarch.
-mmvcle-mno-mvcle- Generate (or do not generate) code using the
mvcle instruction
to perform block moves. When -mno-mvcle is specified,
use a mvc loop instead. This is the default.
-mdebug-mno-debug- Print (or do not print) additional debug information when compiling.
The default is to not print debug information.
-march=cpu-type- Generate code that will run on cpu-type, which is the name of a system
representing a certain processor type. Possible values for
cpu-type are g5, g6, z900, and z990.
When generating code using the instructions available on z/Architecture,
the default is -march=z900. Otherwise, the default is
-march=g5.
-mtune=cpu-type- Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions.
The list of cpu-type values is the same as for -march.
The default is the value used for -march.
-mtpf-trace-mno-tpf-trace- Generate code that adds (does not add) in TPF OS specific branches to trace
routines in the operating system. This option is off by default, even
when compiling for the TPF OS.
-mfused-madd-mno-fused-madd- Generate code that uses (does not use) the floating point multiply and
accumulate instructions. These instructions are generated by default if
hardware floating point is used.
-mwarn-framesize=framesize- Emit a warning if the current function exceeds the given frame size. Because
this is a compile time check it doesn't need to be a real problem when the program
runs. It is intended to identify functions which most probably cause
a stack overflow. It is useful to be used in an environment with limited stack
size e.g. the linux kernel.
-mwarn-dynamicstack- Emit a warning if the function calls alloca or uses dynamically
sized arrays. This is generally a bad idea with a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size- These arguments always have to be used in conjunction. If they are present the s390
back end emits additional instructions in the function prologue which trigger a trap
if the stack size is stack-guard bytes above the stack-size
(remember that the stack on s390 grows downward). These options are intended to
be used to help debugging stack overflow problems. The additionally emitted code
cause only little overhead and hence can also be used in production like systems
without greater performance degradation. The given values have to be exact
powers of 2 and stack-size has to be greater than stack-guard.
In order to be efficient the extra code makes the assumption that the stack starts
at an address aligned to the value given by stack-size.
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