[AMDGPU] GlobalISel fails to select G_TRUNC

[chinmaydd]

Godbolt: https://godbolt.org/z/jEdY3aGWo

IR:

declare i16 @external_i16_func_void()

define amdgpu_kernel void @test_call_external_i16_func_void() {
  %val = call i16 @external_i16_func_void()
  store volatile i16 %val, ptr addrspace(1) poison
  ret void
}

Command:

$ llc -global-isel=1 -mtriple=amdgcn-amd-amdhsa -mcpu=gfx1100 -mattr=-flat-for-global 

Error:

Select:  %13:vgpr_32(s16) = G_TRUNC %26:vgpr(s32)
Failed to constrain G_TRUNC
Selection failed!
Created no instructions.
LLVM ERROR: cannot select: %13:vgpr_32(s16) = G_TRUNC %26:vgpr_32(s32) (in function: test_call_external_i16_func_void)
Compiler returned: 1

The IR is valid and works for global-isel=0 and global-isel=1 but without the -mattr=-flat-for-global restriction.

I also note that the issue is specific to gfx11. I see a crash for gfx12, but that could be related to something else.

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Chinmay Deshpande (chinmaydd)

Godbolt: https://godbolt.org/z/jEdY3aGWo

IR:

declare i16 @<!-- -->external_i16_func_void()

; GCN-LABEL: {{^}}test_call_external_i16_func_void:
define amdgpu_kernel void @<!-- -->test_call_external_i16_func_void() #<!-- -->0 {
  %val = call i16 @<!-- -->external_i16_func_void()
  store volatile i16 %val, ptr addrspace(1) poison
  ret void
}

Command:

$ llc -global-isel=1 -mtriple=amdgcn-amd-amdhsa -mcpu=gfx1100 -mattr=-flat-for-global 

Error:

Select:  %13:vgpr_32(s16) = G_TRUNC %26:vgpr(s32)
Failed to constrain G_TRUNC
Selection failed!
Created no instructions.
LLVM ERROR: cannot select: %13:vgpr_32(s16) = G_TRUNC %26:vgpr_32(s32) (in function: test_call_external_i16_func_void)
Compiler returned: 1

The IR is valid and works for global-isel=0 and global-isel=1 but without the -mattr=-flat-for-global restriction.