Architecture-Specific Options
In addition to the optimization options discussed in the previous section, GCC can generate code specific to each CPU type. To do so, use the -m{value} option. Table 3.6 shows a number of the supported options for the Intel i386 processor family.
Table 3.6 Architecture-Specific GCC Options
|
Option |
Meaning |
|
-mcpu=CPU TYPE |
Uses the default CPU instruction schedule for CPU_TYPE when compiling. The choices for CPU TYPE are i386, i486, i586, pentium, i686, and pentiumpro. |
|
-m386 |
Synonym for -mcpu=i386. |
|
-m486 |
Synonym for -mcpu=i486. |
|
-mpentium |
Synonym for -mcpu=pentium. |
|
-mpentiumpro |
Synonym for -mcpu=pentiumpro. |
|
-march=CPU TYPE |
Generates instructions for CPU TYPE. The choices for CPU TYPE are i386, i486, pentium, and pentiumpro. -march=CPU TYPE implies -mcpu=CPU TYPE. |
|
-mieee-fp |
Uses IEEE standards for floating-point comparisons. |
|
-mno-ieee-fp |
Doesn't use IEEE standards for floating-point comparisons. |
|
-malign-double |
Aligns double, long double, and long long variables on a two word boundary, resulting in faster code. |
|
-mno-align-double |
Doesn't align double, long double, and long long variables on a two word boundary. |
|
-mrtd |
Forces functions that take a fixed number of arguments to return with the ret NUM instruction, saving one instruction in the caller. |
A few notes on these options are necessary. Using a specific CPU TYPE with -mcpu=CPU_TYPE generates appropriate instructions for the indicated CPU. However, you must also use -march=CPU_TYPE or the compiler will generate code that also runs on i386 CPUs. Similarly, to generate code and instruction scheduling specific to a given processor, use the -march=CPU_TYPE option. This is the best way to customize compiled code for a given processor. The -malign_double option results in slightly faster code, as noted in Table 3.6, but it only works for Pentium CPUs. The -mrtd option overrides the -fdefer-stack-pops discussed earlier.