[PATCH] D107187: [amdgpu] Add an enhanced conversion from i64 to f32.

[Jay Foad via Phabricator via llvm-commits]

foad added inline comments.


================
Comment at: llvm/test/CodeGen/AMDGPU/sint_to_fp.i64.ll:233-239
+; GFX8-NEXT:    v_xor_b32_e32 v0, v1, v2
+; GFX8-NEXT:    v_cmp_lt_i32_e32 vcc, -1, v0
+; GFX8-NEXT:    v_ffbh_i32_e32 v5, v2
+; GFX8-NEXT:    v_cndmask_b32_e64 v0, 32, 33, vcc
+; GFX8-NEXT:    v_cmp_ne_u32_e32 vcc, -1, v5
+; GFX8-NEXT:    v_cndmask_b32_e32 v5, v0, v5, vcc
+; GFX8-NEXT:    v_add_u32_e32 v0, vcc, -1, v5
----------------
I think you might be able to shave off another instruction from this sequence with something like:
```
v_alignbit v0, v1, v2, 31 ; extract bits 62..31
v_ashrrev v3, 31, v2 ; duplicate sign bit 32 times
v_xor v0, v0, v3 ; mask is 0 where bits 62..31 match sign bit
v_ffbh_u32 v0, v0 ; count how many of the high bits from 62..31 match the sign bit
v_min_u32 v0, 32, v0 ; clamp to 32
```
Now v0 is the shift amount for the v_lshlrev_b64.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D107187/new/

https://reviews.llvm.org/D107187