Understanding GCC mcpu option

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The GCC option -mcpu= for ARC does not only designate the ARC CPU family (ARC EM, HS, 600 or 700), but also enables the corresponding set of optional instructions defined for the selected configuration. Therefore a particular -mcpu value selects not only a family, but also other -m<something> GCC options for ARC.

Value of -mcpu= option not only sets various other -m<something> options to particular values, but also selects a specific standard library build which was done for this particular core configuration. It is possible to override selection of hardware extensions by passing individual -m<something> options to compiler after the -mcpu= option, however standard library build used for linkage still will be the one matching -mcpu= value. Therefore, for example, option combination -mcpu=em4 -mno-code-density will generate code that doesn't use code density instructions, however it will be linked with standard library that has been built with just -mcpu=em4, which uses code density instructions - therefore final application still may use code density instructions. That's why TCF generator, for example, analyzes hardware features present in the configured processor and selects -mcpu= value that is the best match for this configuration.

ARC EM

The following table summarize what options are set by each of the possible -mcpu values for ARC EM.

-mcpu=	-mcode-density	-mnorm	-mswap	-mbarrel-shifter	-mdiv-rem	-mmpy-option	-mfpu
em						none
em4	Y					none
arcem	Y			Y		wlh1
em4_dmips	Y	Y	Y	Y	Y	wlh1
em4_fpus	Y	Y	Y	Y	Y	wlh1	fpus
em4_fpuda	Y	Y	Y	Y	Y	wlh1	fpuda

The above -mcpu values correspond to specific ARC EM Processor templates presented in the ARChitect tool. It should be noted however that some ARC features are not currently supported in the GNU toolchain, for example DSP instruction support, reduced register size and reduced register sets. Relationship between -mcpu values above and ARC EM Processor templates in ARChitect tool are limited to options listed in the table. Tables will be updated as support for more options get added to the GNU toolchain.

• -mcpu=em doesn't correspond to any specific template, it simply defines the base ARC EM configuration without any optional instructions.
• -mcpu=em4 is a base ARC EM core configuration with -mcode-density option. It corresponds to the following ARC EM templates in ARChitect: em4_mini, em4_sensor, em4_ecc, em6_mini, em5d_mini, em5d_mini_v3, em5d_nrg, em7d_nrg, em9d_mini. Note, however, that those mini templates has a reduced core register file, which is not supported by the GNU toolchain, so application compiled with such TCF or -mcpu=em4 will not run on an nSIM using this TCF - it is requried to add nsim_isa_rgf_num_regs=32 to nSIM options.
• -mcpu=arcem doesn't correspond to any specific template, it is legacy flag preserved for compatibility with older GNU toolchain versions, where -mcpu used to select only a CPU family, while optional features were enabled or disable by individual -m<something> options.
• -mcpu=em4_dmips is a full-featured ARC EM configuration for integer operations. It corresponds to the following ARC EM templates in ARChitect: em4_dmips, em4_rtos, em6_dmips, em4_dmips_v3, em4_parity, em6_dmips_v3, em6_gp, em5d_voice_audio, em5d_nrg_v3, em7d_nrg_v3, em7d_voice_audio, em9d_nrg, em9d_voice_audio, em11d_nrg and em11d_voice_audio.
• -mcpu=em4_fpus is like em4_dmips but with additional support for single-precision floating point unit. It corresponds to the following ARC EM templates in ARChitect: em4_dmips_fpusp, em4_dmips_fpusp_v3, em5d_nrg_fpusp and em9d_nrg_fpusp.
• -mcpu=em4_fpuda is like em4_fpus but with additional support for double-precision assist instructions. It corresponds to the following ARC EM templates in ARChitect: em4_dmips_fpuspdp and em4_dmips_fpuspdp_v3.

ARC HS

The following table summarize what options are set by each of the possible -mcpu values for ARC HS.

-mcpu=	-mdiv-rem	-matomic	-mll64	-mmpy-option	-mfpu
hs		Y		none
hs34		Y		mpy
archs	Y	Y	Y	mpy
hs38	Y	Y	Y	plus_qmacw
hs38_linux	Y	Y	Y	plus_qmacw	fpud_all
hs4x	Y	Y	Y	plus_qmacw
hs4xd	Y	Y	Y	plus_qmacw

The above -mcpu values correspond to specific ARC HS Processor templates presented in the ARChitect tool. It should be noted however that some ARC features are not currently supported in the GNU toolchain, for example reduced register size and reduced register sets. Relationship between -mcpu values above and ARC HS Processor templates in ARChitect tool are limited to options listed in the table. Tables will be updated as support for more options get added to the GNU toolchain.

• -mcpu=hs corresponds to a basic ARC HS with only atomic instructions enabled. It corresponds to the following ARC HS templates in ARChitect: hs34_base, hs36_base and hs38_base.
• -mcpu=hs34 is like hs but with with additional support for standard hardware multiplier. It corresponds to the following ARC HS templates in ARChitect: hs34, hs36 and hs38.
• -mcpu=archs is a generic CPU, which corresponds to the default configuration in older GNU toolchain versions.
• -mcpu=hs38 is a fully featured ARC HS. It corresponds to the following ARC HS templates in ARChitect: hs38_full
• -mcpu=hs38_linux is a fully featured ARC HS with additional support for double-precision FPU. It corresponds to the following ARC HS templates in ARChitect: hs38_slc_full.
• -mcpu=hs4x is like hs38 but employs a different scheduler to take advantage of the delayed multiplier unit, and double integer pipes.
• -mcpu=hs4xd is like hs4x with a tuned scheduler for this cpu.

ARC 600 and ARC 700

The following table summarize what options are set by each of the possible -mcpu values for ARC 600 and ARC 700.

-mcpu	-mnorm	-mswap	-mbarrel-shifter	multiplier
arc700	Y	Y	Y	-mmpy
arc600			Y
arc600_norm	Y		Y
arc600_mul64	Y		Y	-mmul64
arc600_mul32x16	Y		Y	-mmul32x16
arc601
arc601_norm	Y
arc601_mul64	Y			-mmul64
arc601_mul32x16	Y			-mmul32x16