Add RISC-V Profiles format

README.mkd

@@ -64,6 +64,36 @@ specification (input is case sensitive, ...).
 If the 'C' (compressed) instruction set extension is targeted, the compiler 
 will generate compressed instructions where possible.
 
+### Profile-based format
+
+Profiles should be recognized and used in the `-march=` option. The benefit use
+`-march` option is easy for toolchain parsing the profiles string and expanding
+it into normal extensions combinations. 
+
+Profiles format has the following form `-march=<profile-name>[+<option-ext>]+`.
+
+As the spec defines, to use the profiles it should follow profile naming convention
+(See [3.4 form spec doc](https://github.com/riscv/riscv-profiles)), the toolchain
+will check whether an input profile name is correct at first, then do the parse
+work.
+
+In order to distinguish between ordinary extension input and input with profiles,
+profiles are assumed to be entered `at the beginning of the -march option`, and
+then input other extensions. Profiles `should use uppercase letters` in the `-march`
+option.
+
+e.g. `-march = RVA20U64` is a legal profile input, it will be expand into:
+
+`-march = rv64imafdc_zicsr_ziccif_ziccrse_ziccamoa_zicclsm_za128rs`,
+which include all the mandatory extensions required by this profile.
+
+`-march = RVA20U64+zba_zbb_zbc_zbs` is also a legal profile input, it will add
+four new extensions after expanded profile strings:
+
+`-march = rv64imafdc_zicsr_ziccif_ziccrse_ziccamoa_zicclsm_za128rs_zba_zbb_zbc_zbs`
+
+and `-march = rva20u64` is an illegal profile input, it not use uppercase letters.
+
 ### Issues for consideration
 * Whether `riscv32` and `riscv64` should be accepted as synonyms for `rv32` 
 and `rv64`.