Add InstCombineNoArraysPass

This pass wraps LLVM's vanilla InstCombine, to hijack one of its
options. The goal of this hijack is to prevent InstCombine from
splitting and replacing accesses to arrays with GEPs and smaller
accesses.

lib/Support/CustomizedLLVMPasses.cpp

@@ -3,6 +3,8 @@
 //
 
 #include "llvm/Passes/PassBuilder.h"
+#include "llvm/Transforms/InstCombine/InstCombine.h"
+#include "llvm/Transforms/InstCombine/InstCombiner.h"
 #include "llvm/Transforms/Scalar/SROA.h"
 #include "llvm/Transforms/Scalar/SimplifyCFG.h"
 
@@ -83,3 +85,36 @@ public:
 char SROANoArraysPass::ID = 0;
 
 static RegisterPass<SROANoArraysPass> RSROA("sroa-noarrays", "", false, false);
+
+//
+// Customized InstructionCombiningPass, to disable flattening of arrays
+//
+
+class InstCombineNoArrays : public InstructionCombiningPass {
+public:
+  static char ID;
+
+  InstCombineNoArrays() : InstructionCombiningPass() {}
+
+  bool runOnFunction(Function &F) override {
+    // Temporarily swap out the MaxArraySize cl::opt and force it to 0, so that
+    // when the InstCombinePass runs and reads it it doesn't replace loads and
+    // stores to array-typed allocas.
+    // This is a pattern we need to prevent instcombine to optimize away,
+    // because it replaces multi-byte memory accesses with many single-byte
+    // accesses, which is detrimental for information we need to recover about
+    // memory accesses in the analyzed binary program.
+    unsigned OriginalMaxArraySize = MaxArraySize;
+    MaxArraySize = 0;
+
+    InstructionCombiningPass::runOnFunction(F);
+
+    MaxArraySize = OriginalMaxArraySize;
+    return true;
+  }
+};
+
+char InstCombineNoArrays::ID;
+
+using RegisterInstCombine = RegisterPass<InstCombineNoArrays>;
+static RegisterInstCombine RIC("instcombine-noarrays", "", false, false);

share/revng/pipelines/revng-pipelines.yml

@@ -647,7 +647,7 @@ Branches:
               - cleanup-stack-size-markers
               - dce
               - sroa-noarrays
-              - instcombine
+              - instcombine-noarrays
               - remove-extractvalues
               - sroa-noarrays
               - simplify-cfg-with-hoist-and-sink
@@ -660,7 +660,7 @@ Branches:
               # loop-simplify because it kills the loop-simplify form causing
               # DLA not to identify arrays properly
               - loop-simplify
-              - instcombine
+              - instcombine-noarrays
               - remove-extractvalues
               - early-cse
               - dce