[PATCH] D27413: Simplify gep (gep p, a), (b-a)

[Andrea Canciani via Phabricator via llvm-commits]

ranma42 updated this revision to Diff 81542.
ranma42 added a comment.

The new patch extends the patterns recognised by InstCombine as suggested by Eli Friedman.
It can now optimise any gep (gep p, a), b which can be represented as gep p, a+b whenever the a+b operation is folded or simplified away by InstSimplify.
This strictly extends the patterns that are captured, because the previous code only simplified the addition when one operand was 0 or both were constant.


https://reviews.llvm.org/D27413

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/getelementptr.ll


Index: test/Transforms/InstCombine/getelementptr.ll
===================================================================
--- test/Transforms/InstCombine/getelementptr.ll
+++ test/Transforms/InstCombine/getelementptr.ll
@@ -883,6 +883,15 @@
 ; CHECK-NEXT:  ret %struct.C* [[GEP]]
 }
 
+define i32* @test47(i32* %I, i64 %C, i64 %D) {
+  %A = getelementptr i32, i32* %I, i64 %C
+  %sub = sub i64 %D, %C
+  %B = getelementptr i32, i32* %A, i64 %sub
+  ret i32* %B
+; CHECK-LABEL: @test47(
+; CHECK: %B = getelementptr i32, i32* %I, i64 %D
+}
+
 define i32 addrspace(1)* @ascast_0_gep(i32* %p) nounwind {
 ; CHECK-LABEL: @ascast_0_gep(
 ; CHECK-NOT: getelementptr
Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1553,27 +1553,21 @@
       // Replace: gep (gep %P, long B), long A, ...
       // With:    T = long A+B; gep %P, T, ...
       //
-      Value *Sum;
       Value *SO1 = Src->getOperand(Src->getNumOperands()-1);
       Value *GO1 = GEP.getOperand(1);
-      if (SO1 == Constant::getNullValue(SO1->getType())) {
-        Sum = GO1;
-      } else if (GO1 == Constant::getNullValue(GO1->getType())) {
-        Sum = SO1;
-      } else {
-        // If they aren't the same type, then the input hasn't been processed
-        // by the loop above yet (which canonicalizes sequential index types to
-        // intptr_t).  Just avoid transforming this until the input has been
-        // normalized.
-        if (SO1->getType() != GO1->getType())
-          return nullptr;
-        // Only do the combine when GO1 and SO1 are both constants. Only in
-        // this case, we are sure the cost after the merge is never more than
-        // that before the merge.
-        if (!isa<Constant>(GO1) || !isa<Constant>(SO1))
-          return nullptr;
-        Sum = Builder->CreateAdd(SO1, GO1, PtrOp->getName()+".sum");
-      }
+
+      // If they aren't the same type, then the input hasn't been processed
+      // by the loop above yet (which canonicalizes sequential index types to
+      // intptr_t).  Just avoid transforming this until the input has been
+      // normalized.
+      if (SO1->getType() != GO1->getType())
+        return nullptr;
+
+      Value* Sum = SimplifyAddInst(GO1, SO1, false, false, DL, &TLI, &DT, &AC);
+      // Only do the combine when we are sure the cost after the
+      // merge is never more than that before the merge.
+      if (Sum == nullptr)
+        return nullptr;
 
       // Update the GEP in place if possible.
       if (Src->getNumOperands() == 2) {


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