[llvm] d734edf - [MemDep] Reduce block limit

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2023-01-13T10:40:30+01:00
New Revision: d734edfe7c13d1b8e32d75a5df897ef0d9b69302

URL: https://github.com/llvm/llvm-project/commit/d734edfe7c13d1b8e32d75a5df897ef0d9b69302
DIFF: https://github.com/llvm/llvm-project/commit/d734edfe7c13d1b8e32d75a5df897ef0d9b69302.diff

LOG: [MemDep] Reduce block limit

The non-local MemDep analysis has a limit on the number of blocks
it will scan trying to find dependencies. The current limit of 1000
is very high, especially when we consider that each block scan can
also visit up to 100 instructions. In degenerate cases (where we
actually scan that many blocks) MemDep/GVN dominate overall
compile-time, for little benefit.

This patch reduces the limit to 200, which is probably still too
large, but at least mitigates some of the more catastrophic cases.
(For comparison, MSSA clobber walks consider up to 100
MemoryDefs/MemoryPhis, rather than 200 blocks * 100 instructions,
but these limits aren't directly comparable.)

I know that we were kind of hoping that this issue would resolve
itself in time, either by a switch to NewGVN or use of MSSA in GVN.
But I think we should still address this in the meantime.
Additionally, a switch to an MSSA-based implementation will
effectively be doing this as well, in a roundabout way (by dint of
MSSA having lower cutoffs than MDA).

Differential Revision: https://reviews.llvm.org/D140097

Added: 
    

Modified: 
    llvm/lib/Analysis/MemoryDependenceAnalysis.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp b/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
index 2340015d65171..7a13e7a641f32 100644
--- a/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -76,9 +76,9 @@ static cl::opt<unsigned> BlockScanLimit(
              "dependency analysis (default = 100)"));
 
 static cl::opt<unsigned>
-    BlockNumberLimit("memdep-block-number-limit", cl::Hidden, cl::init(1000),
+    BlockNumberLimit("memdep-block-number-limit", cl::Hidden, cl::init(200),
                      cl::desc("The number of blocks to scan during memory "
-                              "dependency analysis (default = 1000)"));
+                              "dependency analysis (default = 200)"));
 
 // Limit on the number of memdep results to process.
 static const unsigned int NumResultsLimit = 100;