[AMDGPU][GISel] Dead code generated by GISel

[tyb0807]

Given this IR, the last basic block is translated with llc -O3 -march=amdgcn -mcpu=gfx942 -print-after-all -mtriple amdgcn-amd-hmcsa -global-isel --asm-verbose --asm-show-inst input.txt -o gisel.s 2> gisel.mir into

 bb.4 (%ir-block.1168):
 ; predecessors: %bb.1
   liveins: $agpr55, $sgpr15, $sgpr22, $vgpr53, $agpr32_agpr33_agpr34_agpr35, $agpr36_agpr37_agpr38_agpr39, $agpr40_agpr41_agpr42_agpr43, $agpr44_agpr45_agpr46_agpr47, $agpr48_agpr49_agpr50_agpr51, $agpr56_agpr57_agpr58_agpr59, $agpr60_agpr61_
   $vgpr16_vgpr17_vgpr18_vgpr19 = SCRATCH_LOAD_DWORDX4_ST 0, 0, implicit $exec, implicit $flat_scr :: (load (s128) from %stack.0, align 4, addrspace 5)
   renamable $vgpr20, dead renamable $sgpr0_sgpr1 = V_DIV_SCALE_F32_e64 0, $vgpr53, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr28 = nofpexcept V_RCP_F32_e32 $vgpr20, implicit $mode, implicit $exec
   renamable $vgpr21, renamable $vcc = V_DIV_SCALE_F32_e64 0, 1065353216, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   $vgpr32 = V_MOV_B32_e32 $vgpr53, implicit $exec, implicit $exec
   renamable $vgpr22 = nofpexcept V_FMA_F32_e64 1, $vgpr20, 0, $vgpr28, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr28 = nofpexcept V_FMAC_F32_e32 killed $vgpr22, killed $vgpr28, $vgpr28(tied-def 0), implicit $mode, implicit $exec
   renamable $vgpr29 = nofpexcept V_MUL_F32_e32 $vgpr21, $vgpr28, implicit $mode, implicit $exec
   renamable $vgpr22 = nofpexcept V_FMA_F32_e64 1, $vgpr20, 0, $vgpr29, 0, $vgpr21, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr29 = nofpexcept V_FMAC_F32_e32 killed $vgpr22, $vgpr28, killed $vgpr29(tied-def 0), implicit $mode, implicit $exec
   renamable $vgpr30 = nofpexcept V_FMA_F32_e64 1, killed $vgpr20, 0, $vgpr29, 0, killed $vgpr21, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr36 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr37 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr38 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr39 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr40 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr41 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr42 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr43 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr44 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr45 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr46 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr47 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr48 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr49 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec
   renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
   renamable $vgpr26 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec
...

Only the result of the first V_DIV_FMAS_F32_e64 is used in later code ($vgpr36), the other copies (e.g. $vgpr37/38/39/...) are not used and these registers are overridden.

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: None (tyb0807)

Given [this IR](https://github.com/user-attachments/files/18398484/input.txt), the last basic block is translated with `llc -O3 -march=amdgcn -mcpu=gfx942 -print-after-all -mtriple amdgcn-amd-hmcsa -global-isel --asm-verbose --asm-show-inst input.txt -o gisel.s 2> gisel.mir` into ``` bb.4 (%ir-block.1168): ; predecessors: %bb.1 liveins: $agpr55, $sgpr15, $sgpr22, $vgpr53, $agpr32_agpr33_agpr34_agpr35, $agpr36_agpr37_agpr38_agpr39, $agpr40_agpr41_agpr42_agpr43, $agpr44_agpr45_agpr46_agpr47, $agpr48_agpr49_agpr50_agpr51, $agpr56_agpr57_agpr58_agpr59, $agpr60_agpr61_ $vgpr16_vgpr17_vgpr18_vgpr19 = SCRATCH_LOAD_DWORDX4_ST 0, 0, implicit $exec, implicit $flat_scr :: (load (s128) from %stack.0, align 4, addrspace 5) renamable $vgpr20, dead renamable $sgpr0_sgpr1 = V_DIV_SCALE_F32_e64 0, $vgpr53, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr28 = nofpexcept V_RCP_F32_e32 $vgpr20, implicit $mode, implicit $exec renamable $vgpr21, renamable $vcc = V_DIV_SCALE_F32_e64 0, 1065353216, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec $vgpr32 = V_MOV_B32_e32 $vgpr53, implicit $exec, implicit $exec renamable $vgpr22 = nofpexcept V_FMA_F32_e64 1, $vgpr20, 0, $vgpr28, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr28 = nofpexcept V_FMAC_F32_e32 killed $vgpr22, killed $vgpr28, $vgpr28(tied-def 0), implicit $mode, implicit $exec renamable $vgpr29 = nofpexcept V_MUL_F32_e32 $vgpr21, $vgpr28, implicit $mode, implicit $exec renamable $vgpr22 = nofpexcept V_FMA_F32_e64 1, $vgpr20, 0, $vgpr29, 0, $vgpr21, 0, 0, implicit $mode, implicit $exec renamable $vgpr29 = nofpexcept V_FMAC_F32_e32 killed $vgpr22, $vgpr28, killed $vgpr29(tied-def 0), implicit $mode, implicit $exec renamable $vgpr30 = nofpexcept V_FMA_F32_e64 1, killed $vgpr20, 0, $vgpr29, 0, killed $vgpr21, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr36 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr37 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr38 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr39 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr40 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr41 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr42 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr43 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr44 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr45 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr46 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr47 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr48 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr49 = nofpexcept V_DIV_FIXUP_F32_e64 0, killed $vgpr31, 0, $vgpr53, 0, 1065353216, 0, 0, implicit $mode, implicit $exec renamable $vgpr31 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec renamable $vgpr26 = nofpexcept V_DIV_FMAS_F32_e64 0, $vgpr30, 0, $vgpr28, 0, $vgpr29, 0, 0, implicit $mode, implicit $vcc, implicit $exec ... ``` Only the result of the first `V_DIV_FMAS_F32_e64` is used in later code (`$vgpr36`), the other copies (e.g. `$vgpr37/38/39/...`) are not used and these registers are overridden.

[qcolombet]

I'm looking into this and there are several issues here, not sure about DCE in particular, but we miss some CSE for sure.
That being said, if I run instcombine and early-use on the input it removes some of the problems.

My take is SDISel has been historically more aggressive with instcombine type of optimization whereas in GISel we tend to rely more on the previous LLVM IR passes to avoid code duplication in the selector itself and have a policy closer to garbage in garbage out.
That doesn't mean we shouldn't fix it, but that's probably lower priority.

I'll file separate issues with what I find.