fallback: legalize: G_CTLZ_ZERO

fallback: legalize: G_CTLZ_ZERO_POISON s32,s128

Run:	explore-04ca53e65f-20261014-114124
Kind:	fallback
Category:	legalize: G_CTLZ_ZERO_POISON s32,s128
Occurrences:	9 in 6 configs, 4 test files
Snippet from:	idiv_large.ll @udiv_i128 via llvm-extract
Config:	`-mtriple=riscv32`
Local check:	reproduces at 04ca53e65f
Compiler Explorer:	not uploaded (over --godbolt-limit or disabled)

Snippet:

; Function Attrs: nounwind
define i128 @udiv_i128(i128 %x, i128 %y) #0 {
  %res = udiv i128 %x, %y
  ret i128 %res
}

attributes #0 = { nounwind }

Reproduce:

llc -mtriple=riscv32 -global-isel -global-isel-abort=2 -pass-remarks-missed=gisel.* snippet.ll -o -
llc -mtriple=riscv32 -global-isel=0 snippet.ll -o -

Local llc output:

remark: <unknown>:0:0: unable to legalize instruction: %1031:_(s32) = G_CTLZ_ZERO_POISON %6:_(s128) (in function: udiv_i128)
warning: Instruction selection used fallback path for udiv_i128

Occurrences (9):

Input	Function	Config
idiv_large.ll	udiv_i128	`-mtriple=riscv32`
abds.d7ddfb2137.ll (mutant of abds.ll)	abd_select_i128	`-mtriple=riscv32`
calling-conv-ilp32e.042503f8c6.ll (mutant of calling-conv-ilp32e.ll)	callee_large_scalars_exhausted_regs	`-mtriple=riscv32 -target-abi ilp32e -frame-pointer=all -verify-machineinstrs`
calling-conv-ilp32e.655fc67c2e.ll (mutant of calling-conv-ilp32e.ll)	callee_large_scalars_exhausted_regs	`-mtriple=riscv32 -target-abi ilp32e -mattr=+save-restore -frame-pointer=all -verify-machineinstrs`
calling-conv-ilp32e.7e34e49a95.ll (mutant of calling-conv-ilp32e.ll)	callee_large_scalars_exhausted_regs	`-mtriple=riscv32 -target-abi ilp32e -mattr=+save-restore -verify-machineinstrs`
calling-conv-ilp32e.84f9ae9fb6.ll (mutant of calling-conv-ilp32e.ll)	callee_large_scalars_exhausted_regs	`-mtriple=riscv32 -target-abi ilp32e -verify-machineinstrs`
idiv_large.7e7f05fb17.ll (mutant of idiv_large.ll)	udiv_i128	`-mtriple=riscv32`
idiv_large.c72c6bad34.ll (mutant of idiv_large.ll)	udiv_i128	`-mtriple=riscv32`
shifts.b84dee6549.ll (mutant of shifts.ll)	ashr128	`-mtriple=riscv32 -verify-machineinstrs`

Generated 2026-10-15 08:05 UTC.