urem+udiv miscompile with global isel on arm64

[regehr]

this is being miscompiled by the arm64 backend with global isel:

define i32 @test1(i32 %0) {
  %2 = urem i32 %0, %0
  %3 = udiv i32 %2, %0
  ret i32 %3
}

we want it to just return 0 (or something else that always produces 0) but we get:

regehr@john-home:~$ llc foo.ll -o - -global-isel --march=arm64
	.text
	.file	"foo.ll"
	.globl	test1                           // -- Begin function test1
	.p2align	2
	.type	test1,@function
test1:                                  // @test1
	.cfi_startproc
// %bb.0:
	udiv	w0, w0, w0
	ret
.Lfunc_end0:
	.size	test1, .Lfunc_end0-test1
	.cfi_endproc
                                        // -- End function
	.section	".note.GNU-stack","",@progbits
regehr@john-home:~$ 

cc @ornata @nunoplopes @ryan-berger @nbushehri @zhengyang92 @aqjune @Hatsunespica

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

[regehr]

we're also seeing versions of this bug with signed operations

regehr@john-home:~$ cat foo.ll
define i8 @test_sdiv8(i8 %0, i8 %1) #0 {
  %3 = srem i8 %1, %1
  %4 = sdiv i8 %3, %1
  ret i8 %4
}
regehr@john-home:~$ llc foo.ll -o - -global-isel --march=arm64
	.text
	.file	"foo.ll"
	.globl	test_sdiv8                      // -- Begin function test_sdiv8
	.p2align	2
	.type	test_sdiv8,@function
test_sdiv8:                             // @test_sdiv8
	.cfi_startproc
// %bb.0:
	sxtb	w8, w1
	sdiv	w0, w8, w8
	ret
.Lfunc_end0:
	.size	test_sdiv8, .Lfunc_end0-test_sdiv8
	.cfi_endproc
                                        // -- End function
	.section	".note.GNU-stack","",@progbits
regehr@john-home:~$ 

[nicolas17]

Testing the original code in compiler explorer:

define i32 @test1(i32 %0) {
  %2 = urem i32 %0, %0
  %3 = udiv i32 %2, %0
  ret i32 %3
}

LLVM 11 outputs:

        udiv    w8, w0, w0
        mul     w8, w8, w0
        subs    w8, w0, w8
        udiv    w0, w8, w0
        ret

LLVM 12 outputs:

        udiv    w8, w0, w0
        msub    w8, w8, w0, w0
        udiv    w0, w8, w0
        ret

I think both of these are correct and always result in 0.

LLVM 13 outputs:

        udiv    w0, w0, w0
        ret

So, it's bisect time...

[nicolas17]

Bisect result:

In 4c6ab48, instead of emitting the udiv+msub+udiv posted above, it started failing altogether with LLVM ERROR: unable to legalize instruction: %2:_(s32), %1:_ = G_UDIVREM %0:_, %0:_ (in function: test1).

In ac1647c (3986 commits later), it stopped erroring out in the legalizer and started emitting udiv w0, w0, w0.

[ornata]

cc @TNorthover

[aemerson]

https://reviews.llvm.org/D130525 and committed in 5ae0472