Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1348,6 +1348,11 @@
         // normalized.
         if (SO1->getType() != GO1->getType())
           return nullptr;
+          
+        // Is it worth summing the offsets?
+        if (!isa<ConstantInt>(SO1))
+        	return nullptr;
+        
         Sum = Builder->CreateAdd(SO1, GO1, PtrOp->getName()+".sum");
       }
 
Index: test/Transforms/InstCombine/getelementptr.ll
===================================================================
--- test/Transforms/InstCombine/getelementptr.ll
+++ test/Transforms/InstCombine/getelementptr.ll
@@ -98,15 +98,44 @@
   ret void
 }
 
+; Ensure we don't canonicalize chains of GEPs with many unknowns into arithmetic
 define i32* @test7(i32* %I, i64 %C, i64 %D) {
         %A = getelementptr i32* %I, i64 %C
         %B = getelementptr i32* %A, i64 %D
         ret i32* %B
 ; CHECK-LABEL: @test7(
-; CHECK: %A.sum = add i64 %C, %D
-; CHECK: getelementptr i32* %I, i64 %A.sum
+; CHECK: getelementptr i32* %I, i64 %C
+; CHECK: getelementptr i32* %A, i64 %D
 }
 
+; Don't canonicalize away from (%stack - %offset) + idx form (#19973)
+define i32 @test_gep_canon_1(i32* %stack, i64 %offset) {
+        %neg = sub i64 0, %offset
+        %base = getelementptr i32* %stack, i64 %neg
+        %1 = getelementptr i32* %base, i64 1
+        %2 = getelementptr i32* %base, i64 2
+        %A = load i32* %1
+        %B = load i32* %2
+        %ret = add i32 %A, %B
+        ret i32 %ret
+  ; CHECK-LABEL: @test_gep_canon_1(
+  ; CHECK: %neg = sub i64 0, %offset
+  ; CHECK-NEXT: getelementptr i32* %stack, i64 %neg
+  ; CHECK: getelementptr i32* %base, i64 1
+  ; CHECK: getelementptr i32* %base, i64 2
+}
+
+; Fold constant offsets hidden in arithmetic
+define i32* @test_gep_canon_2_add(i32* %A, i32 %B) {
+        %C = add i32 %B, 3
+        %D = getelementptr i32* %A, i32 %C
+        %E = getelementptr i32* %D, i32 4
+        ret i32* %E
+  ; CHECK-LABEL: @test_gep_canon_2_add(
+  ; CHECK: %D = getelementptr i32* %A, i32 %B
+  ; CHECK: getelementptr i32* %D, i32 7
+}
+
 define i8* @test8([10 x i32]* %X) {
         ;; Fold into the cast.
         %A = getelementptr [10 x i32]* %X, i64 0, i64 0