Missing Support for ARM Floating Point vFp5 Spec

[haydenridd]

The Cortex M7 chip (ISA v7-EM) from ARM supports the "FPv5" floating point spec:
https://developer.arm.com/documentation/ddi0489/f/floating-point-unit/about-the-fpu

Just built llvm from release tag 18.1.7 and ran the following:

llvm-project/build/bin/llvm-tblgen --dump-json llvm-project/llvm/lib/Target/ARM/ARM.td -I=llvm-project/llvm/include -I=llvm-project/llvm/lib/Target/ARM > ARM_TARGETS.json

To verify which options were supported.

Floating point features for the Cortex M7 (and in general) only go up to vfp4.

ARM chips are backwards compatible with previous floating point versions, so it's valid to use vfp4 for the Cortex-M7, however it caused me a good deal of confusion when porting a project from GCC to find vfp5 nowhere in sight!

For context this was discovered because:

• Zig uses llvm-tblgen to parse + populate some enums that define valid LLVM features per CPU
• I am developing with Zig + a Cortex M7 based MCU, and was confused why vfp5 wasn't in the available CPU features

[llvmbot]

@llvm/issue-subscribers-backend-arm

Author: None (haydenridd)

The Cortex M7 chip (ISA v7-EM) from ARM supports the "FPv5" floating point spec: https://developer.arm.com/documentation/ddi0489/f/floating-point-unit/about-the-fpu

Just built llvm from release tag 18.1.7 and ran the following:

llvm-project/build/bin/llvm-tblgen --dump-json llvm-project/llvm/lib/Target/ARM/ARM.td -I=llvm-project/llvm/include -I=llvm-project/llvm/lib/Target/ARM > ARM_TARGETS.json

To verify which options were supported.

Floating point features for the Cortex M7 (and in general) only go up to vfp4.

ARM chips are backwards compatible with previous floating point versions, so it's valid to use vfp4 for the Cortex-M7, however it caused me a good deal of confusion when porting a project from GCC to find vfp5 nowhere in sight!

For context this was discovered because:

• Zig uses llvm-tblgen to parse + populate some enums that define valid LLVM features per CPU
• I am developing with Zig + a Cortex M7 based MCU, and was confused why vfp5 wasn't in the available CPU features

[davemgreen]

Hi. My understanding is that fpv5 is the same as FPARMv8 in terms of instructions, so they share the same feature names. The clang mappings can be seen in

llvm-project/llvm/include/llvm/TargetParser/ARMTargetParser.def

Line 48 in e9a3623

ARM_FPU("fp-armv8", FK_FP_ARMV8, FPUVersion::VFPV5, NeonSupportLevel::None,

.
Cortex-M7 uses FK_FPV5_D16.

[haydenridd]

Ahh, amazing thank you! I spent far too much time reading past LLVM PR's only to confuse myself more about whether or not these were the same :) Closing!