[ARM] Incorrect __gnu_h2f_ieee ABI

[nikic]

target triple = "arm-unknown-linux-gnueabihf"

define i1 @test(half %self) #0 {
  %_0 = fcmp une half %self, %self
  ret i1 %_0
}

attributes #0 = { "target-features"="+vfp2" }

Produces:

	.save	{r11, lr}
	push	{r11, lr}
	vmov	r0, s0
	bl	__gnu_h2f_ieee
	vcmp.f32	s0, s0
	mov	r0, #0
	vmrs	APSR_nzcv, fpscr
	movvs	r0, #1
	pop	{r11, lr}
	mov	pc, lr

LLVM 20 produced the correct:

	.save	{r11, lr}
	push	{r11, lr}
	vmov	r0, s0
	bl	__gnu_h2f_ieee
	vmov	s0, r0
	mov	r0, #0
	vcmp.f32	s0, s0
	vmrs	APSR_nzcv, fpscr
	movvs	r0, #1
	pop	{r11, lr}
	mov	pc, lr

[llvmbot]

@llvm/issue-subscribers-backend-arm

Author: Nikita Popov (nikic)

```llvm target triple = "arm-unknown-linux-gnueabihf"

define i1 @test(half %self) #0 {
%_0 = fcmp une half %self, %self
ret i1 %_0
}

attributes #0 = { "target-features"="+vfp2" }

Produces:
```asm
	.save	{r11, lr}
	push	{r11, lr}
	vmov	r0, s0
	bl	__gnu_h2f_ieee
	vcmp.f32	s0, s0
	mov	r0, #<!-- -->0
	vmrs	APSR_nzcv, fpscr
	movvs	r0, #<!-- -->1
	pop	{r11, lr}
	mov	pc, lr

LLVM 20 produced the correct:

	.save	{r11, lr}
	push	{r11, lr}
	vmov	r0, s0
	bl	__gnu_h2f_ieee
	vmov	s0, r0
	mov	r0, #<!-- -->0
	vcmp.f32	s0, s0
	vmrs	APSR_nzcv, fpscr
	movvs	r0, #<!-- -->1
	pop	{r11, lr}
	mov	pc, lr

[nikic]

@arsenm This issue was introduced in the RuntimeLibCalls migration.

From a quick look

llvm-project/llvm/lib/Target/ARM/ARMISelLowering.cpp

Lines 710 to 723 in a446300

	// The half <-> float conversion functions are always soft-float on
	// non-watchos platforms, but are needed for some targets which use a
	// hard-float calling convention by default.
	if (!TT.isWatchABI()) {
	if (TM.isAAPCS_ABI()) {
	setLibcallImplCallingConv(RTLIB::__truncsfhf2, CallingConv::ARM_AAPCS);
	setLibcallImplCallingConv(RTLIB::__truncdfhf2, CallingConv::ARM_AAPCS);
	setLibcallImplCallingConv(RTLIB::__extendhfsf2, CallingConv::ARM_AAPCS);
	} else {
	setLibcallImplCallingConv(RTLIB::__truncsfhf2, CallingConv::ARM_APCS);
	setLibcallImplCallingConv(RTLIB::__truncdfhf2, CallingConv::ARM_APCS);
	setLibcallImplCallingConv(RTLIB::__extendhfsf2, CallingConv::ARM_APCS);
	}
	}

now specifies AAPCS on __extendhfsf2 specifically, while previously it was on all FPEXT_F16_F32 libcalls, including __gnu_h2f_ieee.