[llvm] AMDGPU: Make v2f32 -> v2f16 legal when target supports v_cvt_pk_f16_f32 (PR #139956)

[Changpeng Fang via llvm-commits]

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@@ -12,6 +12,20 @@ define <2 x half> @v_test_cvt_v2f32_v2f16(<2 x float> %src) {
   ret <2 x half> %res
 }
 
+define half @fptrunc_v2f32_v2f16_then_extract(<2 x float> %src) {
+; GFX950-LABEL: fptrunc_v2f32_v2f16_then_extract:
+; GFX950:       ; %bb.0:
+; GFX950-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
+; GFX950-NEXT:    v_cvt_pk_f16_f32 v0, v0, v1
+; GFX950-NEXT:    v_add_f16_sdwa v0, v0, v0 dst_sel:DWORD dst_unused:UNUSED_PAD src0_sel:WORD_1 src1_sel:DWORD
+; GFX950-NEXT:    s_setpc_b64 s[30:31]
+  %vec_half = fptrunc <2 x float> %src to <2 x half>
+  %first = extractelement <2 x half> %vec_half, i64 1
+  %second = extractelement <2 x half> %vec_half, i64 0
+  %res = fadd half %first, %second
+  ret half %res
+}
+
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changpeng wrote:

No. This change only takes care of v2f16. It is beyond this work to test wider vectors.
Additional work is needed for wider vector splitting (in order to generate the packed conversions)

https://github.com/llvm/llvm-project/pull/139956