Roman Lebedev c7933992b4 [InstCombine] Try to reuse constant from select in leading comparison ... Summary: If we have e.g.: ``` %t = icmp ult i32 %x, 65536 %r = select i1 %t, i32 %y, i32 65535 ``` the constants `65535` and `65536` are suspiciously close. We could perform a transformation to deduplicate them: ``` Name: ult %t = icmp ult i32 %x, 65536 %r = select i1 %t, i32 %y, i32 65535 => %t.inv = icmp ugt i32 %x, 65535 %r = select i1 %t.inv, i32 65535, i32 %y ``` https://rise4fun.com/Alive/avb While this may seem esoteric, this should certainly be good for vectors (less constant pool usage) and for opt-for-size - need to have only one constant. But the real fun part here is that it allows further transformation, in particular it finishes cleaning up the `clamp` folding, see e.g. `canonicalize-clamp-with-select-of-constant-threshold-pattern.ll`. We start with e.g. ``` %dont_need_to_clamp_positive = icmp sle i32 %X, 32767 %dont_need_to_clamp_negative = icmp sge i32 %X, -32768 %clamp_limit = select i1 %dont_need_to_clamp_positive, i32 -32768, i32 32767 %dont_need_to_clamp = and i1 %dont_need_to_clamp_positive, %dont_need_to_clamp_negative %R = select i1 %dont_need_to_clamp, i32 %X, i32 %clamp_limit ``` without this patch we currently produce ``` %1 = icmp slt i32 %X, 32768 %2 = icmp sgt i32 %X, -32768 %3 = select i1 %2, i32 %X, i32 -32768 %R = select i1 %1, i32 %3, i32 32767 ``` which isn't really a `clamp` - both comparisons are performed on the original value, this patch changes it into ``` %1.inv = icmp sgt i32 %X, 32767 %2 = icmp sgt i32 %X, -32768 %3 = select i1 %2, i32 %X, i32 -32768 %R = select i1 %1.inv, i32 32767, i32 %3 ``` and then the magic happens! Some further transform finishes polishing it and we finally get: ``` %t1 = icmp sgt i32 %X, -32768 %t2 = select i1 %t1, i32 %X, i32 -32768 %t3 = icmp slt i32 %t2, 32767 %R = select i1 %t3, i32 %t2, i32 32767 ``` which is beautiful and just what we want. Proofs for `getFlippedStrictnessPredicateAndConstant()` for de-canonicalization: https://rise4fun.com/Alive/THl Proofs for the fold itself: https://rise4fun.com/Alive/THl Reviewers: spatel, dmgreen, nikic, xbolva00 Reviewed By: spatel Subscribers: hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D66232 llvm-svn: 369840		2019-08-24 06:49:25 +00:00
..
AggressiveInstCombine
Coroutines	[llvm] Migrate llvm::make_unique to std::make_unique	2019-08-15 15:54:37 +00:00
Hello
InstCombine	[InstCombine] Try to reuse constant from select in leading comparison	2019-08-24 06:49:25 +00:00
Instrumentation	hwasan: Fix use of uninitialized memory.	2019-08-23 21:37:20 +00:00
IPO	[Attributor] Manifest alignment in load and store instructions	2019-08-23 20:20:10 +00:00
ObjCARC	[ObjC][ARC] Delete ObjC runtime calls on global variables annotated	2019-06-14 22:06:32 +00:00
Scalar	[LoopFusion] Fix -Wunused-function in -DLLVM_ENABLE_ASSERTIONS=off build	2019-08-24 02:50:42 +00:00
Utils	[Loop Peeling] Fix silly bug in metadata update.	2019-08-22 10:06:46 +00:00
Vectorize	[SLP] use range-for loops, fix formatting; NFC	2019-08-23 16:22:32 +00:00
CMakeLists.txt
LLVMBuild.txt