[llvm] Deal vector data failed when using option Os、-mfloat-abi=softfp、--target=armebv7-linux

[Zhenhang1213]

I find results in armebv7-linux are different from armv7.
code

#include <stdio.h> 
volatile int one = 1;

int main (int argc, char *argv[]) {
    __attribute__((vector_size((8)*sizeof(short)))) short v0 = {one, 1, 2, 3, 4, 5, 6, 7};
    __attribute__((vector_size((8)*sizeof(short)))) short v1;
    v1 = v0 % 2; 
    for (int __i = 0; __i < 8; __i++) { 
        printf("v1: %d\n", (*((short *) &(v1) + __i))); 
    }
    return 0;
}

When __i is 4, the result is different. I think this Instruction: vrev32.16 d17, d18 is wrong, and I try to modify it is vmov.16 d17, d18, the result is right. However I don't know how to fix this bug.

https://godbolt.org/z/EqYebY73z

[llvmbot]

@llvm/issue-subscribers-backend-arm

Author: Austin (Zhenhang1213)

I find results in armebv7-linux are different from armv7. code ''' #include <stdio.h> volatile int one = 1;

int main (int argc, char *argv[]) {
attribute((vector_size((8)*sizeof(short)))) short v0 = {one, 1, 2, 3, 4, 5, 6, 7};
attribute((vector_size((8)sizeof(short)))) short v1;
v1 = v0 % 2;
for (int __i = 0; __i < 8; __i++) {
printf("v1: %d\n", (((short *) &(v1) + __i)));
}
return 0;
}
'''

When __i is 4, the result is different. I think this Instruction: vrev32.16 d17, d18 is wrong, and I try to modify it is vmov.16 d17, d18, the result is right. However I don't know how to fix this bug.

https://godbolt.org/z/EqYebY73z

[davemgreen]

#97782 recently fixed a similar sounding issue, but it doesn't look like the code has changed on trunk https://godbolt.org/z/7WYn38xTs.

[Zhenhang1213]

#97782 recently fixed a similar sounding issue, but it doesn't look like the code has changed on trunk https://godbolt.org/z/7WYn38xTs.

yes, I compile clang with this patch, and the code is same

[hstk30-hw]

Same problem, but different situation.
The vrev32.16 d17, d18 is unnecessary,

vmov.i32        d18, #0x10000
vrev32.16 d17, d18

repalce to

vmov.i32        d17, #0x10000

is ok.

[Zhenhang1213]

Same problem, but different situation. The vrev32.16 d17, d18 is unnecessary,

vmov.i32        d18, #0x10000
vrev32.16 d17, d18

repalce to

vmov.i32        d17, #0x10000

is ok.

@davemgreen could you give me some ideas? I've been puzzled by this question for a long time.

[davemgreen]

Hi it honestly looks like vector constants are generated incorrectly if they can be specified with a larger movi size (i.e. using a vmov.i32 for a i16 constant splat).
https://godbolt.org/z/oPczbxbvq

That may be related to #68212, we perhaps should be passing IsBigEndian to isConstantSplat in LowerBUILD_VECTOR. The bitcast of the VMOVIMM also looks suspicious, and there might be a problem in PerformBITCASTCombine where it is ignoring casts that it should not. There might be cases where we get it wrong in two places that usually cancel each other out.

[Zhenhang1213]

Hi it honestly looks like vector constants are generated incorrectly if they can be specified with a larger movi size (i.e. using a vmov.i32 for a i16 constant splat). https://godbolt.org/z/oPczbxbvq

That may be related to #68212, we perhaps should be passing IsBigEndian to isConstantSplat in LowerBUILD_VECTOR. The bitcast of the VMOVIMM also looks suspicious, and there might be a problem in PerformBITCASTCombine where it is ignoring casts that it should not. There might be cases where we get it wrong in two places that usually cancel each other out.

However，I find llvm 15 also is wrong without this patch

[Zhenhang1213]

Hi it honestly looks like vector constants are generated incorrectly if they can be specified with a larger movi size (i.e. using a vmov.i32 for a i16 constant splat). https://godbolt.org/z/oPczbxbvq

That may be related to #68212, we perhaps should be passing IsBigEndian to isConstantSplat in LowerBUILD_VECTOR. The bitcast of the VMOVIMM also looks suspicious, and there might be a problem in PerformBITCASTCombine where it is ignoring casts that it should not. There might be cases where we get it wrong in two places that usually cancel each other out.

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

Lines 7966 to 7969 in 6f6422f

	if (Val.getNode()) {
	SDValue Vmov = DAG.getNode(ARMISD::VMOVIMM, dl, VmovVT, Val);
	return DAG.getNode(ISD::BITCAST, dl, VT, Vmov);
	}

Is this code for endianness adjustment? In this scenario, should I adjust the lower part and adjust byte order finally after CONCAT_VECTORS?

[davemgreen]

I don't believe that #68212 broke or fixed anything on it's own as we don't set the new parameter. We might need to add the IsBigEndian parameter with the correct value in the call to isConstantSplat in ARMISelLowering. Then change the BITCAST you point to above into a VECTOR_REG_CAST and then see what else changes and if there are any other follow-on fixes that are needed. From what I remember from looking into it the PerformBITCASTCombine function might need to be tightened-up when there is BITCAST(VECTOR_REG_CAST). The important thing is to test it to make sure we are now getting the right results.

Does that sound do-able? Big-endian is unfortunately much less used than the rest so doesn't get as much testing.

[Zhenhang1213]

I don't believe that #68212 broke or fixed anything on it's own as we don't set the new parameter. We might need to add the IsBigEndian parameter with the correct value in the call to isConstantSplat in ARMISelLowering. Then change the BITCAST you point to above into a VECTOR_REG_CAST and then see what else changes and if there are any other follow-on fixes that are needed. From what I remember from looking into it the PerformBITCASTCombine function might need to be tightened-up when there is BITCAST(VECTOR_REG_CAST). The important thing is to test it to make sure we are now getting the right results.

Does that sound do-able? Big-endian is unfortunately much less used than the rest so doesn't get as much testing.

yes，I try to mody it and some tests failed， I think there are some problems with the modified behavior, such as this case

; RUN: llc -mtriple armeb-eabi -mattr=armv8.2-a,neon,fullfp16 -target-abi=aapcs-gnu -float-abi hard -o - %s | FileCheck %s
define void @conv_v4i16_to_v4f16( <4 x i16> %a, <4 x half>* %store ) {
; CHECK-LABEL: conv_v4i16_to_v4f16:
; CHECK:       @ %bb.0: @ %entry
; CHECK-NEXT:    vmov.i64 d16, #0xffff00000000ffff
; CHECK-NEXT:    vldr d17, [r0]
; CHECK-NEXT:    vrev64.16 d18, d0
; CHECK-NEXT:    vrev64.16 d17, d17
; CHECK-NEXT:    vrev64.16 d16, d16
; CHECK-NEXT:    vadd.i16 d16, d18, d16
; CHECK-NEXT:    vadd.f16 d16, d16, d17
; CHECK-NEXT:    vrev64.16 d16, d16
; CHECK-NEXT:    vstr d16, [r0]
; CHECK-NEXT:    bx lr
entry:
  %c = add <4 x i16> %a, <i16 -1, i16 0, i16 0, i16 -1>
  %v = bitcast <4 x i16> %c to <4 x half>
  %w = load <4 x half>, <4 x half>* %store
  %z = fadd <4 x half> %v, %w
  store <4 x half> %z, <4 x half>* %store
  ret void
}

and the result is

 vmov.i64        d16, #0xffff00000000ffff
        vldr    d17, [r0]
        vrev64.16       d18, d0
        vadd.i16        d16, d18, d16
        vrev64.16       d17, d17
        vadd.f16        d16, d16, d17
        vrev64.16       d16, d16
        vstr    d16, [r0]
        bx      lr

The logic of the two parts is not the same. When I debug PerformVECTOR_REG_CASTCombine function， Op->getOpcode() isnot ARMISD::VECTOR_REG_CAST, so return SDValue();

[Zhenhang1213]

I don't believe that #68212 broke or fixed anything on it's own as we don't set the new parameter. We might need to add the IsBigEndian parameter with the correct value in the call to isConstantSplat in ARMISelLowering. Then change the BITCAST you point to above into a VECTOR_REG_CAST and then see what else changes and if there are any other follow-on fixes that are needed. From what I remember from looking into it the PerformBITCASTCombine function might need to be tightened-up when there is BITCAST(VECTOR_REG_CAST). The important thing is to test it to make sure we are now getting the right results.

Does that sound do-able? Big-endian is unfortunately much less used than the rest so doesn't get as much testing.

After adding the IsBigEndian parameter with the correct value in the call to isConstantSplat in ARMISelLowering, cancel the adaptation for 64-bit VMOV of BigEndian in isVMOVModifiedImm

entry:
  %0 = bitcast <4 x i32> %src to <16 x i8>
  %r = and <16 x i8> %0, <i8 0, i8 0, i8 0, i8 1, i8 0, i8 0, i8 0, i8 1, i8 0, i8 0, i8 0, i8 1, i8 0, i8 0, i8 0, i8 1>
  ret <16 x i8> %r

and the instruction generated I think is incorrect，

vrev64.8        q1, q0
vmov.i32        q0, #0x1
vrev32.8        q0, q0
vand            q1, q1, q0
vrev64.8        q0, q1
bx              lr

could you give me more advice? Modifying BITCAST to VECTOR_REG_CAST) brings more failures, thx.

[davemgreen]

I was taking another look and using the IsBigEndian parameter to isConstantSplat might have been a mistake to suggest. I think it would make more sense to always have the constants produced in the "vector-lane-order" (same as little-endian, lowest lane at the bottom), which means not passing isBigEndian through to isConstantSplat.

The constants we create then don't need to be bitcast to change the lane order, they just need to be vector_reg_cast. Apparently i64 are for some reason special and the vector elements get reversed in

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

Line 7124 in dd90c72

if (DAG.getDataLayout().isBigEndian()) {

, that should be removed so we are consistent about which order lanes appear.

Could you put a patch together with those changes? We can go through the test differences to makes sure they all seem correct. I'm pretty sure that something will need to change in PerformBITCASTCombine in the optimization of bitcast(vector_reg_cast(vmovimm)).