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On Freitag 29 Mai 2009, Graham Murray wrote: |
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> Stroller <stroller@××××××××××××××××××.uk> writes: |
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> > But, surely "-march=" also instructs gcc to support the additional |
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> > instructions. Suggest you re-read Daniel's post that I was replying |
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> > to. |
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> > |
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> > What's the difference between supporting the "certain set of |
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> > instructions" with "-march=" and doing so with USEs? |
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> > |
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> > Or doesn't "-march=" support additional "certain sets of |
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> > instructions". What does it do, then? |
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> |
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> I am not sure, |
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> |
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> $ gcc -Q --help=target -march=core2 |
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> The following options are target specific: |
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> -m128bit-long-double [disabled] |
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> -m32 [enabled] |
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> -m3dnow [disabled] |
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> -m3dnowa [disabled] |
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> -m64 [disabled] |
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> -m80387 [enabled] |
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> -m96bit-long-double [enabled] |
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> -mabm [disabled] |
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> -maccumulate-outgoing-args [disabled] |
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> -maes [disabled] |
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> -malign-double [disabled] |
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> -malign-functions= |
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> -malign-jumps= |
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> -malign-loops= |
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> -malign-stringops [enabled] |
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> -march= core2 |
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> -masm= |
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> -mavx [disabled] |
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> -mbranch-cost= |
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> -mcld [disabled] |
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> -mcmodel= |
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> -mcx16 [disabled] |
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> -mfancy-math-387 [enabled] |
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> -mfma [disabled] |
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> -mforce-drap [disabled] |
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> -mfp-ret-in-387 [enabled] |
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> -mfpmath= |
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> -mfused-madd [enabled] |
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> -mglibc [enabled] |
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> -mhard-float [enabled] |
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> -mieee-fp [enabled] |
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> -mincoming-stack-boundary= |
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> -minline-all-stringops [disabled] |
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> -minline-stringops-dynamically [disabled] |
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> -mintel-syntax [disabled] |
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> -mlarge-data-threshold= |
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> -mmmx [disabled] |
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> -mms-bitfields [disabled] |
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> -mno-align-stringops [disabled] |
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> -mno-fancy-math-387 [disabled] |
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> -mno-fused-madd [disabled] |
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> -mno-push-args [disabled] |
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> -mno-red-zone [disabled] |
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> -mno-sse4 [enabled] |
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> -momit-leaf-frame-pointer [disabled] |
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> -mpc |
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> -mpclmul [disabled] |
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> -mpopcnt [disabled] |
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> -mpreferred-stack-boundary= |
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> -mpush-args [enabled] |
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> -mrecip [disabled] |
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> -mred-zone [enabled] |
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> -mregparm= |
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> -mrtd [disabled] |
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> -msahf [disabled] |
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> -msoft-float [disabled] |
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> -msse [disabled] |
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> -msse2 [disabled] |
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> -msse2avx [disabled] |
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> -msse3 [disabled] |
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> -msse4 [disabled] |
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> -msse4.1 [disabled] |
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> -msse4.2 [disabled] |
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> -msse4a [disabled] |
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> -msse5 [disabled] |
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> -msseregparm [disabled] |
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> -mssse3 [disabled] |
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> -mstack-arg-probe [disabled] |
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> -mstackrealign [enabled] |
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> -mstringop-strategy= |
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> -mtls-dialect= |
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> -mtls-direct-seg-refs [enabled] |
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> -mtune= |
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> -muclibc [disabled] |
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> -mveclibabi= |
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|
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and man gcc says: |
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--help={class|[^]qualifier}[,...] |
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Print (on the standard output) a description of the command line |
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options understood by the compiler that fit into all specified |
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classes and qualifiers. These are the supported classes: |
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|
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target |
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This will display target-specific options. Unlike the --target-help |
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option however, target-specific options of the linker |
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and assembler will not be displayed. This is because those |
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tools do not currently support the extended --help= syntax. |
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|
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Which leads to the conclusion, that it only shows options that can be set. Not |
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options how they are really set (besides a few that are enabled by the |
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architecture). If you leave -march=core2 out. you will probably get the same |
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result. |
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|
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for example: |
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-mfpmath=unit |
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Generate floating point arithmetics for selected unit unit. The |
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choices for unit are: |
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|
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387 Use the standard 387 floating point coprocessor present |
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majority of chips and emulated otherwise. Code compiled with this |
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option will run almost everywhere. The temporary results are |
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computed in 80bit precision instead of precision specified by |
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the type resulting in slightly different results compared to |
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most of other chips. See -ffloat-store for more detailed |
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description. |
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|
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This is the default choice for i386 compiler. |
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|
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sse Use scalar floating point instructions present in the SSE |
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instruction set. This instruction set is supported by Pentium3 |
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and newer chips, in the AMD line by Athlon-4, Athlon-xp and |
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Athlon-mp chips. The earlier version of SSE instruction set |
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supports only single precision arithmetics, thus the double and |
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extended precision arithmetics is still done using 387. |
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Later version, present only in Pentium4 and the future AMD |
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x86-64 chips supports double precision arithmetics too. |
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|
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For the i386 compiler, you need to use -march=cpu-type, -msse |
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or -msse2 switches to enable SSE extensions and make this |
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option effective. For the x86-64 compiler, these extensions |
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are enabled by default. |
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|
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The resulting code should be considerably faster in the |
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majority of cases and avoid the numerical instability problems of |
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387 code, but may break some existing code that expects |
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temporaries to be 80bit. |
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|
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This is the default choice for the x86-64 compiler. |
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|
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as you can see from the man excerpt, if the help showed enabled options, |
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mfpmath=sse should be there. It isn't. |
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|
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... |
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