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Mirror of https://github.com/RPCS3/llvm-mirror
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*** Context *** Prior to this patchw, the table generated for matching instruction was straight forward but highly inefficient. Basically, each pattern generates its own set of self contained checks and actions. E.g., TableGen generated: // First pattern CheckNumOperand 3 CheckOpcode G_ADD ... Build ADDrr // Second pattern CheckNumOperand 3 CheckOpcode G_ADD ... Build ADDri // Third pattern CheckNumOperand 3 CheckOpcode G_SUB ... Build SUBrr *** Problem *** Because of that generation, a *lot* of check were redundant between each pattern and were checked every single time until we reach the pattern that matches. E.g., Taking the previous table, let say we are matching a G_SUB, that means we were going to check all the rules for G_ADD before looking at the G_SUB rule. In particular we are going to do: check 3 operands; PASS check G_ADD; FAIL ; Next rule check 3 operands; PASS (but we already knew that!) check G_ADD; FAIL (well it is still not true) ; Next rule check 3 operands; PASS (really!!) check G_SUB; PASS (at last :P) *** Proposed Solution *** This patch introduces a concept of group of rules (GroupMatcher) that share some predicates and only get checked once for the whole group. This patch only creates groups with one nesting level. Conceptually there is nothing preventing us for having deeper nest level. However, the current implementation is not smart enough to share the recording (aka capturing) of values. That limits its ability to do more sharing. For the given example the current patch will generate: // First group CheckOpcode G_ADD // First pattern CheckNumOperand 3 ... Build ADDrr // Second pattern CheckNumOperand 3 ... Build ADDri // Second group CheckOpcode G_SUB // Third pattern CheckNumOperand 3 ... Build SUBrr But if we allowed several nesting level, it could create a sub group for the checknumoperand 3. (We would need to call optimizeRules on the rules within a group.) *** Result *** With only one level of nesting, the instruction selection pass is up to 4x faster. For instance, one instruction now takes 500 checks, instead of 24k! With more nesting we could get in the tens I believe. Differential Revision: https://reviews.llvm.org/D39034 rdar://problem/34670699 llvm-svn: 321017 |
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