[PATCH] Infer known bits from dominating conditions

Philip Reames listmail at philipreames.com
Tue Feb 17 15:59:51 PST 2015

Hi nlewycky, hfinkel,

This patch adds limited support in ValueTracking for inferring known bits of a value from conditional expressions which must be true to reach the instruction we're trying to optimize.  At this time, the feature is off by default.  Once landed, I'm hoping for feedback from others on both profitability and compile time impact.

Forms of conditional value propagation have been tried in LLVM before and have failed due to compile time problems.  In an attempt to side step that, this patch only considers conditions where the edge leaving the branch dominates the context instruction.  Rather than walking back along the dominator tree for every value/context instruction pair, we only consider uses of the value itself when searching for useful conditions.  It's also limited to only the first level call in the known bits recursion search.

Even with these restrictions, it handles many interesting cases:
- Early exits from functions
- Early exits from loops (for context instructions in the loop and after the check)
- Conditions which control entry into loops, including multi-version loops (such as those produced during vectorization, IRCE, loop unswitch, etc..)

Possible applications include optimizing using information provided by constructs such as: preconditions, assumptions, null checks, & range checks.  

When compiling sqlite3.c (using a Release+Asserts build), I get the following timings:
-instcombine -value-tracking-dom-conditions=1: 0.788s-0.793
-instcombine -value-tracking-dom-conditions=0: 0.771-0.773
-O3 -value-tracking-dom-conditions=1: 14.993-15.391
-O3 -value-tracking-dom-conditions=0: 14.961-14.984

There is some slightly slow down in instcombine, but the difference in the O3 times are essentially noise.  I measured an outlier (which I discarded in the above) of 16.408 for "-O3 -value-tracking-dom-conditions=0".



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