integer vector miscompile from instcombine

[regehr]

I believe this is being mis-optimized:

define <4 x i32> @f(<4 x i32> %0) {
  %2 = icmp eq <4 x i32> %0, zeroinitializer
  %3 = shufflevector <4 x i32> %0, <4 x i32> zeroinitializer, <4 x i32> <i32 1, i32 1, i32 0, i32 3>
  %4 = select <4 x i1> %2, <4 x i32> %3, <4 x i32> zeroinitializer
  ret <4 x i32> %4
}

I'll just give Alive's reasoning, which is pretty easy to follow and seems correct:

regehr@ohm:~$ ~/alive2-regehr/build/alive-tv -passes=instcombine foo.ll

----------------------------------------
define <4 x i32> @f(<4 x i32> %#0) {
#1:
  %#2 = icmp eq <4 x i32> %#0, { 0, 0, 0, 0 }
  %#3 = shufflevector <4 x i32> %#0, <4 x i32> { 0, 0, 0, 0 }, 1, 1, 0, 3
  %#4 = select <4 x i1> %#2, <4 x i32> %#3, <4 x i32> { 0, 0, 0, 0 }
  ret <4 x i32> %#4
}
=>
define <4 x i32> @f(<4 x i32> %#0) {
#1:
  ret <4 x i32> { 0, 0, 0, 0 }
}
Transformation doesn't verify!

ERROR: Value mismatch

Example:
<4 x i32> %#0 = < #x00000000 (0), #x00000001 (1), #x00000000 (0), poison >

Source:
<4 x i1> %#2 = < #x1 (1), #x0 (0), #x1 (1), poison >
<4 x i32> %#3 = < #x00000001 (1), #x00000001 (1), #x00000000 (0), poison >
<4 x i32> %#4 = < #x00000001 (1), #x00000000 (0), #x00000000 (0), poison >

Target:
Source value: < #x00000001 (1), #x00000000 (0), #x00000000 (0), poison >
Target value: < #x00000000 (0), #x00000000 (0), #x00000000 (0), #x00000000 (0) >

Summary:
  0 correct transformations
  1 incorrect transformations
  0 failed-to-prove transformations
  0 Alive2 errors
regehr@ohm:~$ 

cc @nunoplopes

[nikic]

Probably caused by 77eb056. Fell into the vector select trap again.