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Fix DCE of FREE of COALESCE
When encountering the following SSA graph: BB1: #2.T1 [string] = COALESCE #1.CV0($str) [null, string] BB2 BB2: #5.T1 [string] = QM_ASSIGN string("") BB3: #7.X1 [string] = Phi(#2.X1 [string], #5.X1 [string]) FREE #7.T1 [string] We would currently determine that #7, #5 are dead, and eliminate the FREE and QM_ASSIGN. However, we cannot eliminate #2, as COALESCE is also responsible for control flow. Fix this my marking all non-CV phis as live to start with. This can be relaxed to check the kind of the source instruction, but I couldn't immediately come up with a case where it would be useful.
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@ -503,6 +503,16 @@ int dce_optimize_op_array(zend_op_array *op_array, zend_ssa *ssa, zend_bool reor
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ctx.phi_dead = alloca(sizeof(zend_ulong) * ctx.phi_worklist_len);
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memset(ctx.phi_dead, 0xff, sizeof(zend_ulong) * ctx.phi_worklist_len);
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/* Mark non-CV phis as live. Even if the result is unused, we generally cannot remove one
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* of the producing instructions, as it combines producing the result with control flow.
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* This can be made more precise if there are any cases where this is not the case. */
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FOREACH_PHI(phi) {
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if (phi->var >= op_array->last_var) {
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zend_bitset_excl(ctx.phi_dead, phi->ssa_var);
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add_phi_sources_to_worklists(&ctx, phi, 0);
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}
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} FOREACH_PHI_END();
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/* Mark reacable instruction without side effects as dead */
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int b = ssa->cfg.blocks_count;
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while (b > 0) {
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17
ext/opcache/tests/opt/dce_009.phpt
Normal file
17
ext/opcache/tests/opt/dce_009.phpt
Normal file
@ -0,0 +1,17 @@
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--TEST--
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Incorrect DCE of FREE of COALESCE
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--FILE--
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<?php
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function test(?string $str) {
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$str ?? $str = '';
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return strlen($str);
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}
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$foo = 'foo';
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$foo .= 'bar';
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var_dump(test($foo));
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?>
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--EXPECT--
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int(6)
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