GlobalISel: infinite loop trying to legalize G_BUILD_VECTOR

[jayfoad]

; RUN: llc -global-isel=1 -mtriple=amdgcn -mcpu=gfx1030 < %s
define void @main(<19 x i32> %arg) {
bb:
  %i = bitcast <19 x i32> %arg to <38 x i16>
  %i1 = extractelement <38 x i16> %i, i64 0
  %i2 = icmp eq i16 %i1, 0
  %i3 = zext i1 %i2 to i32
  %i4 = bitcast i32 %i3 to float
  %i5 = insertelement <4 x float> zeroinitializer, float %i4, i64 0
  call void @llvm.amdgcn.image.store.2d.v4f32.i32(<4 x float> %i5, i32 0, i32 0, i32 0, <8 x i32> zeroinitializer, i32 0, i32 0)
  ret void
}
declare void @llvm.amdgcn.image.store.2d.v4f32.i32(<4 x float>, i32 immarg, i32, i32, <8 x i32>, i32 immarg, i32 immarg)

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Jay Foad (jayfoad)

``` ; RUN: llc -global-isel=1 -mtriple=amdgcn -mcpu=gfx1030 < %s define void @main(<19 x i32> %arg) { bb: %i = bitcast <19 x i32> %arg to <38 x i16> %i1 = extractelement <38 x i16> %i, i64 0 %i2 = icmp eq i16 %i1, 0 %i3 = zext i1 %i2 to i32 %i4 = bitcast i32 %i3 to float %i5 = insertelement <4 x float> zeroinitializer, float %i4, i64 0 call void @llvm.amdgcn.image.store.2d.v4f32.i32(<4 x float> %i5, i32 0, i32 0, i32 0, <8 x i32> zeroinitializer, i32 0, i32 0) ret void } declare void @llvm.amdgcn.image.store.2d.v4f32.i32(<4 x float>, i32 immarg, i32, i32, <8 x i32>, i32 immarg, i32 immarg) ```

[petar-avramovic]

The problem here is that
%0:_(<19 x s32>) = G_BUILD_VECTOR is not legal for <19 x s32>
while the user of %0:_(<19 x s32>) is legal for <19 x s32>. User is G_BITCAST

  %20:_(<38 x s16>) = G_BITCAST %0:_(<19 x s32>)
  %32:_(<19 x s32>) = G_BITCAST %20:_(<38 x s16>)

G_BUILD_VECTOR waits for user to get legalized, legalization process should create G_UNMERGE_VALUES (<19 x s32>) and this two should be combined away.
Note: artifact combiner is not allowed to look through G_BITCAST so G_BITCAST has to be legalized by the target.
The not allowed to look through comes from how G_BITCAST was described in globalisel documentation if I remember correctly.

In summary G_BITCAST should not be legal for <19 x s32> and <38 x s16>

Good first step is to first properly list all all legal types.
There is a PR that tried to implement this
#68189