Make activity analysis more aggressive re gep (rust-lang#761)

Author: wsmoses

enzyme/Enzyme/ActivityAnalysis.cpp

@@ -2483,6 +2483,13 @@ bool ActivityAnalyzer::isValueInactiveFromUsers(TypeResults const &TR,
       }
     }
 
+    // For an inbound gep, args which are not the pointer being offset
+    // are not used in an active way by definition.
+    if (auto gep = dyn_cast<GetElementPtrInst>(a)) {
+      if (gep->isInBounds() && gep->getPointerOperand() != parent)
+        continue;
+    }
+
     // If this doesn't write to memory this can only be an active use
     // if its return is used in an active way, therefore add this to
     // the list of users to analyze

enzyme/test/ActivityAnalysis/ashrgep.ll

@@ -0,0 +1,31 @@
+; RUN: %opt < %s %loadEnzyme -print-activity-analysis -activity-analysis-func=f -enzyme-strict-aliasing=0 -o /dev/null | FileCheck %s
+
+declare i64 @getint(double* %x)
+
+define void @f(double* %x) {
+entry:
+  %a189 = call i64 @getint(double* %x)
+  %a190 = ashr exact i64 %a189, 4
+  br i1 true, label %r, label %e
+
+r:
+  %g = getelementptr inbounds double, double* %x, i64 %a190
+  store double 1.000000e+00, double* %g, align 8
+  br label %e
+
+e:
+  ret void
+}
+
+; CHECK: double* %x: icv:0
+; CHECK-NEXT: entry
+; CHECK-NEXT:   %a189 = call i64 @getint(double* %x): icv:0 ici:0
+; CHECK-NEXT:   %a190 = ashr exact i64 %a189, 4: icv:0 ici:1
+; CHECK-NEXT:   br i1 true, label %r, label %e: icv:1 ici:1
+; CHECK-NEXT: r
+; CHECK-NEXT:   %g = getelementptr inbounds double, double* %x, i64 %a190: icv:0 ici:1
+; CHECK-NEXT:   store double 1.000000e+00, double* %g, align 8: icv:1 ici:1
+; CHECK-NEXT:   br label %e: icv:1 ici:1
+; CHECK-NEXT: e
+; CHECK-NEXT:   ret void: icv:1 ici:1
+