Float16T mismatch casts (LLVM-325)

[kassane]

I was trying to understand the Xtensa floating point alignment, but I got an llvm-ir error about converting floatFromInt on esp32|esp32s3|cnl targets, only esp32s2 had no problem.

I could work around this problem (not solved) by removing the fp (Single FP) and s32c1i or by removing the compiler-rt.

I didn't see any reference to f16 or u16 in the Xtensa target in the llvm-17.0.1 source.

This issue only occurs when creating an executable. Unlike static objects and libraries.

Reference

• LLVM ERROR: floatFromInt cast kassane/zig-espressif-bootstrap#2

[gerekon]

Hi @kassane !

What llvm-project branch do you use? LLVM 17.x release branch? I suppose that floating point support has not been upstreamed yet. You can try using our repo instead (https://github.com/espressif/llvm-project/tree/xtensa_release_17.0.1).
@andreisfr Am I correct?

[kassane]

I'm using this branch exactly xtensa_release_17.0.1.

[andreisfr]

@kassane , thank you very much for report, we will investigate the problem.

[kassane]

Do you know if xtensa enables conversion intrinsic fp16?

Missing on:
https://github.com/espressif/llvm-project/blob/xtensa_release_17.0.1/clang/lib/Basic/Targets/Xtensa.h

bool useFP16ConversionIntrinsics() const override { return /*false or true?*/; }

search link: https://github.com/search?q=repo%3Aespressif%2Fllvm-project%20FP16Conversion&type=code

[gerekon]

@kassane I am looking into this issue.
Could you try with #92?

[kassane]

Could you try with #92?

Hi @gerekon,
I had tried before, but the error still persists. XtensaISD::PCREL_WRAPPER TargetConstantPool:i32<i32 31744>

LLVM Emit Object... LLVM ERROR: Cannot select: 0x7d7a4f7d0770: f32 = fp16_to_fp 0x7d7a50359440, float_from_int.zig:46:9 @[ floatsihf.zig:11:24 ]
  0x7d7a50359440: i32 = or 0x7d7a4f84cfc0, 0x7d7a503598a0, float_from_int.zig:46:9 @[ floatsihf.zig:11:24 ]
    0x7d7a4f84cfc0: i32 = AssertZext 0x7d7a50359d70, ValueType:ch:i16, float_from_int.zig:46:9 @[ floatsihf.zig:11:24 ]
      0x7d7a50359d70: i32,ch = CopyFromReg 0x7d7a4f9973e0, Register:i32 %1, float_from_int.zig:46:9 @[ floatsihf.zig:11:24 ]
        0x7d7a4f0c40b0: i32 = Register %1
    0x7d7a503598a0: i32 = XtensaISD::PCREL_WRAPPER TargetConstantPool:i32<i32 31744> 0
      0x7d7a4f0ca650: i32 = TargetConstantPool<i32 31744> 0
In function: __floatsihf
``

[gerekon]

ok. Sorry for confusion. Do you have a simple scenario to reproduce this?

[kassane]

ok. Sorry for confusion. Do you have a simple scenario to reproduce this?

No worries!
For testing, try use: https://github.com/kassane/zig-espressif-bootstrap/releases

Make anywhere freestanding example for make executable.

void _start(){}

Command:

# CPU = esp8266**|esp32s3|esp32|esp32s2*|cnl
# *: no issue
# **: no tested yet

# clang cli
zig cc start.c -target xtensa-freestanding-none -mcpu=CPU
# or zig cli
zig build-exe start.c -target xtensa-freestanding-none -mcpu=CPU
# if add -fno-compiler-rt no issue for build-exe or build-lib -dynamic
# replace build-exe to build-lib [staticlib, default] or build-obj, no get error (no compiler-rt by default)

[gerekon]

Ok. Thanks.
Looks like I managed to reproduce this with Clang clang -O0 --target=xtensa-esp-elf -mcpu=esp32 fp16_test.c

static volatile __fp16 s_val;

int main()
{
  int ival = 5;
  s_val = ival;
  return 0;
}

and got

fatal error: error in backend: Cannot select: 0x5e8bc8788e80: ch = store<(volatile store (s16) into @s_val), trunc to f16> 0x5e8bc8788a90:1, 0x5e8bc8788b00, 0x5e8bc8788be0, undef:i32
  0x5e8bc8788b00: f32 = sint_to_fp 0x5e8bc8788a90
    0x5e8bc8788a90: i32,ch = load<(dereferenceable load (s32) from %ir.2)> 0x5e8bc8788a20, FrameIndex:i32<1>, undef:i32
      0x5e8bc87889b0: i32 = FrameIndex<1>
      0x5e8bc8788860: i32 = undef
  0x5e8bc8788be0: i32 = XtensaISD::PCREL_WRAPPER TargetConstantPool:i32<@s_val = internal global half 0xH0000, align 2> 0
    0x5e8bc8788b70: i32 = TargetConstantPool<@s_val = internal global half 0xH0000, align 2> 0
  0x5e8bc8788860: i32 = undef
In function: main
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)

Looks like the same problem.

[gerekon]

@andreisfr @sstefan1 Do you have any idea?

[gerekon]

@maciej-czekaj Added floating support in #89. And have float intrinsics
https://github.com/espressif/llvm-project/blob/xtensa_release_17.0.1/llvm/include/llvm/IR/IntrinsicsXtensa.td#L253.
Smth wrong happens with int<->f16 conversion.

[gerekon]

@kassane Ok. Looks like I made a step toward solution. Now @llvm.convert.to.fp16, @llvm.convert.from.fp16 are used for converting. This is default behaviour expanding to calls to __gnu_f2h_ieee and __gnu_h2f_ieee from compiler-rt. Looks like the code missed actions for fp32<->fp16 conversion. But in my example I have linker problems

ld.lld: error: /mnt/host/projects/esp/tmp/build_esp_clang/llvm/bin/../lib/clang-runtimes/xtensa-esp-unknown-elf/esp32/lib/libclang_rt.builtins.a(truncsfhf2.c.obj):(function __truncsfhf2: .text.__truncsfhf2+0xb6): relocation R_XTENSA_SLOT0_OP out of range: 100 is not in [-262141, 18446744073709551612]; references section '.literal.__truncsfhf2'

But that can be specific to my toolchain build only.
I have not tried your scenario yet, so maybe in your case everything works with this changes :-).

But in any case implemented fix makes use of software conversion functions. In future we will need to lower this to HW instructions if possible.

[kassane]

This is default behaviour expanding to calls to __gnu_f2h_ieee and __gnu_h2f_ieee

Last commit I attempted to turn false gnu_fp16 (similar to riscv32), but didn't get a different result.
https://github.com/kassane/zig/blob/528e739220f091d420f3f693d23c1c29d909dc85/lib/compiler_rt/common.zig#L49C1-L65
While the F16T function xtensa jumps in the else to u16 instead of f16.
https://github.com/kassane/zig/blob/528e739220f091d420f3f693d23c1c29d909dc85/lib/compiler_rt/common.zig#L85C8-L85C12

I have linker problems

Indeed. lld (builtin), zig doesn't allow -fuse-ld= (only -fno-lld [switch to zld]).

But in any case implemented fix makes use of software conversion functions. In future we will need to lower this to HW instructions if possible.

Ok.

[gerekon]

Indeed. lld (builtin), zig doesn't allow -fuse-ld= (only -fno-lld [switch to zld]).

In our toolchain problem is solved using GNU ld by passing --ld-path= to clang. Switching to LLD is in plans.

[kassane]

@gerekon ,

Before applying your current patch, the new error mentioned in the PR occurred when removing the -fp extension. (current toolchain available on releases)

# -mcpu=<esp32|esp32s3>-fp
error: <unknown>:0: Undefined temporary symbol .LBB385_2
# -mcpu=esp32-fp-s32c1i
lld errors: undefined references...

Your patch applied:
-mcpu=esp32-s32c1i
lld errors: undefined references...