<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Issue</th>
<td>
<a href=https://github.com/llvm/llvm-project/issues/122905>122905</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>
[GISel] Missing combines to expose common subexpressions and eliminate them
</td>
</tr>
<tr>
<th>Labels</th>
<td>
llvm:globalisel
</td>
</tr>
<tr>
<th>Assignees</th>
<td>
</td>
</tr>
<tr>
<th>Reporter</th>
<td>
qcolombet
</td>
</tr>
</table>
<pre>
When running GISel on the given input IR, we end up generating essentially a one-to-one mapping with the input IR whereas most of the computation is just duplicated.
SDISel performs a much better job by doing `instcombine`-like optimizations that allow it to simplify the IR and eventually exposes the CSE opportunity.
This is not that surprising given that historically GISel has had a garbage in garbage out approach, but it may make sense to strengthen GISel combines for optimized builds.
Note: I observed this with AMDGPU but I suspect it affects all backends.
# To Reproduce #
Download the attached IR or copy paste the snippet below and run
```bash
llc -O3 -march=amdgcn -mcpu=gfx942 -mtriple amdgcn-amd-hmcsa -global-isel=<0|1> repro.ll -o <sd|g>isel.s
```
# Result #
GISel ends up generating two `fdiv` instructions whereas SDISel is able to CSE the whole thing and produces just one.
GISel:
```asm
v_div_scale_f32 v1, s[0:1], v0, v0, 1.0
v_mov_b32_e32 v7, v2
v_mov_b32_e32 v2, v3
v_mov_b32_e32 v3, v4
v_rcp_f32_e32 v4, v1
v_div_scale_f32 v5, vcc, 1.0, v0, 1.0
v_fma_f32 v8, -v1, v4, 1.0
v_fmac_f32_e32 v4, v8, v4
v_mul_f32_e32 v8, v5, v4
v_fma_f32 v9, -v1, v8, v5
v_fmac_f32_e32 v8, v9, v4
v_fma_f32 v1, -v1, v8, v5
v_div_fmas_f32 v5, v1, v4, v8
v_div_fixup_f32 v5, v5, v0, 1.0
v_div_fmas_f32 v1, v1, v4, v8
v_div_fixup_f32 v0, v1, v0, 1.0
```
SDISel:
```asm
v_div_scale_f32 v1, s[0:1], v0, v0, 1.0
v_rcp_f32_e32 v6, v1
s_nop 0
v_fma_f32 v7, -v1, v6, 1.0
v_fmac_f32_e32 v6, v7, v6
v_div_scale_f32 v7, vcc, 1.0, v0, 1.0
v_mul_f32_e32 v8, v7, v6
v_fma_f32 v9, -v1, v8, v7
v_fmac_f32_e32 v8, v9, v6
v_fma_f32 v1, -v1, v8, v7
v_div_fmas_f32 v1, v1, v6, v8
v_div_fixup_f32 v0, v1, v0, 1.0
```
# IR Snippet #
```llvm
define void @foo(<1 x float> %in, ptr %out, ptr %out2) {
%t1174 = insertvalue [4 x <1 x float>] zeroinitializer, <1 x float> %in, 0
%t1175 = insertvalue [4 x <1 x float>] %t1174, <1 x float> %in, 1
%t1176 = insertvalue [4 x <1 x float>] %t1175, <1 x float> %in, 2
%t1177 = insertvalue [4 x <1 x float>] %t1176, <1 x float> %in, 3
%t1178 = insertvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] zeroinitializer, [4 x <1 x float>] %t1177, 0, 0, 0, 0
%t1179 = insertvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] %t1178, [4 x <1 x float>] %t1177, 0, 0, 1, 0
%t1180 = insertvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] %t1179, [4 x <1 x float>] %t1177, 0, 0, 2, 0
%t1181 = insertvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] %t1180, [4 x <1 x float>] %t1177, 0, 0, 3, 0
%t1182 = insertvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] %t1181, [4 x <1 x float>] %t1177, 1, 0, 0, 0
%t1183 = insertvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] %t1182, [4 x <1 x float>] %t1177, 1, 0, 1, 0
%t1184 = insertvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] %t1183, [4 x <1 x float>] %t1177, 1, 0, 2, 0
%t1185 = insertvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] %t1184, [4 x <1 x float>] %t1177, 1, 0, 3, 0
%t1186 = extractvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] %t1178, 0, 0, 0, 0, 0
%t1187 = fdiv <1 x float> splat (float 1.000000e+00), %t1186
%t1188 = extractvalue [2 x [1 x [4 x [1 x [4 x <1 x float>]]]]] %t1178, 0, 0, 0, 0, 1
%t1189 = fdiv <1 x float> splat (float 1.000000e+00), %t1188
store <1 x float> %t1187, ptr %out
store <1 x float> %t1189, ptr %out2
ret void
}
```
# Note #
SDISel is essentially able to do the equivalent of:
```bash
opt -passes=instcombine,early-cse -S repro.ll -o -
```
Which yields:
```llvm
define void @foo(<1 x float> %in, ptr %out, ptr %out2) {
%t1187 = fdiv <1 x float> splat (float 1.000000e+00), %in
store <1 x float> %t1187, ptr %out, align 4
store <1 x float> %t1187, ptr %out2, align 4
ret void
}
```
[repro.ll.txt](https://github.com/user-attachments/files/18411275/repro.ll.txt)
</pre>
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