[clang] [flang] [flang][Driver] Support -print-supported-extensions (PR #117402)

[Tarun Prabhu via cfe-commits]

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@@ -0,0 +1,10 @@
+! Test that --print-supported-extensions errors on unsupported architectures.
+
+! REQUIRES: x86-registered-target
+
+! RUN: not %flang --target=x86_64-linux-gnu --print-supported-extensions \
+! RUN:   2>&1 | FileCheck %s
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tarunprabhu wrote:

As I understand it, yes, this option is not available for X86. 

RISC-V publishes [extensions](https://lf-riscv.atlassian.net/wiki/spaces/HOME/pages/16154732/Ratified+Extensions) to the Instruction Set Architecture (ISA). The [usage guide](https://llvm.org/docs/RISCVUsage.html#extensions) describes what LLVM currently supports. As far as I am aware, such a thing does not exist on X86.

The [documentation](https://github.com/llvm/llvm-project/blob/db6f627f3fd4072fe1814805653a352694527a91/clang/include/clang/Driver/Options.td#L5913) for the command line switch also explicitly restricts the option to AArch64, Arm and RISC-V.

https://github.com/llvm/llvm-project/pull/117402