Merge similar if-statements
Submitted by Ian Romanick
Assigned to Ian Romanick
Description
I recently encountered some generated NIR like:
vec1 ssa_0 = load_const (0x00000000 /* 0.000000 */)
...
/* succs: block_3 block_4 */
if ssa_122 {
block block_3:
/* preds: block_2 */
/* succs: block_5 */
} else {
block block_4:
/* preds: block_2 */
vec1 ssa_139 = fmul ssa_138, ssa_1
/* succs: block_5 */
}
block block_5:
/* preds: block_3 block_4 */
vec1 ssa_140 = phi block_3: ssa_0, block_4: ssa_139
vec1 ssa_141 = fadd ssa_17, ssa_140
/* succs: block_6 block_7 */
if ssa_122 {
block block_6:
/* preds: block_5 */
vec1 ssa_142 = fmul ssa_138, ssa_1
/* succs: block_8 */
} else {
block block_7:
/* preds: block_5 */
/* succs: block_8 */
}
block block_8:
/* preds: block_6 block_7 */
vec1 ssa_143 = phi block_6: ssa_142, block_7: ssa_0
The i965 backend generates the obvious, terrible code for this. There are two levels of optimization that could occur on just the NIR. First, merge the two if-statements.
/* succs: block_3 block_4 */
if ssa_122 {
block block_3:
/* preds: block_2 */
vec1 ssa_142 = fmul ssa_138, ssa_1
/* succs: block_5 */
} else {
block block_4:
/* preds: block_2 */
vec1 ssa_139 = fmul ssa_138, ssa_1
/* succs: block_5 */
}
block block_5:
/* preds: block_3 block_4 */
vec1 ssa_140 = phi block_3: ssa_0, block_4: ssa_139
vec1 ssa_143 = phi block_6: ssa_142, block_7: ssa_0
vec1 ssa_141 = fadd ssa_17, ssa_140
Second, notice that the calculations in both branches is the same, and pull it out.
vec1 ssa_999 = fmul ssa_138, ssa_1
/* succs: block_3 block_4 */
if ssa_122 {
block block_3:
/* preds: block_2 */
/* succs: block_5 */
} else {
block block_4:
/* preds: block_2 */
/* succs: block_5 */
}
block block_5:
/* preds: block_3 block_4 */
vec1 ssa_140 = phi block_3: ssa_0, block_4: ssa_999
vec1 ssa_143 = phi block_6: ssa_999, block_7: ssa_0
vec1 ssa_141 = fadd ssa_17, ssa_140
Existing optimization passes should turn this into a pair of bcsel instructions.
Version: git