diff --git a/llvm/test/Analysis/lit.local.cfg b/llvm/test/Analysis/lit.local.cfg
deleted file mode 100644
index 07c3132153e73a0f361369f0dda46d29986c0233..0000000000000000000000000000000000000000
--- a/llvm/test/Analysis/lit.local.cfg
+++ /dev/null
@@ -1,9 +0,0 @@
-# -*- Python -*- vim: set ft=python ts=4 sw=4 expandtab tw=79:
-from lit.llvm.subst import ToolSubst
-
-fc = ToolSubst('FileCheck', unresolved='fatal')
-# Insert this first. Then, we'll first update the blank FileCheck command; then,
-# the default substitution of FileCheck will replace it to its full path.
-config.substitutions.insert(0, (fc.regex, 
-    'FileCheck --allow-unused-prefixes=false'))
-
diff --git a/llvm/test/CodeGen/lit.local.cfg b/llvm/test/CodeGen/lit.local.cfg
deleted file mode 100644
index 2499077fdcd71516d31c0b70000597e7b99fc5e5..0000000000000000000000000000000000000000
--- a/llvm/test/CodeGen/lit.local.cfg
+++ /dev/null
@@ -1,7 +0,0 @@
-from lit.llvm.subst import ToolSubst
-
-fc = ToolSubst('FileCheck', unresolved='fatal')
-# Insert this first. Then, we'll first update the blank FileCheck command; then,
-# the default substitution of FileCheck will replace it to its full path.
-config.substitutions.insert(0, (fc.regex,
-    'FileCheck --allow-unused-prefixes=false'))
diff --git a/llvm/test/FileCheck/lit.local.cfg b/llvm/test/FileCheck/lit.local.cfg
index 9164f683fc1be3197554c22893aebc802936202e..74e4d89e416b265cd17e6093c0fda1d7279ace06 100644
--- a/llvm/test/FileCheck/lit.local.cfg
+++ b/llvm/test/FileCheck/lit.local.cfg
@@ -54,3 +54,12 @@ config.test_format = lit.formats.ShTest(execute_external=False)
 # that test results throughout all test suites are affected.
 config.substitutions.append(('%ProtectFileCheckOutput',
     'env -u FILECHECK_OPTS'))
+
+# FIXME: remove this once the default is flipped.
+from lit.llvm.subst import ToolSubst
+
+fc = ToolSubst('FileCheck', unresolved='fatal')
+# the parent introduced the opposite rule, so we replace it if we see it.
+if len(config.substitutions) > 0 and config.substitutions[0] == (fc.regex, 'FileCheck --allow-unused-prefixes=false'):
+    del config.substitutions[0]
+
diff --git a/llvm/test/Instrumentation/AddressSanitizer/global_metadata_array.ll b/llvm/test/Instrumentation/AddressSanitizer/global_metadata_array.ll
index f5b9e4c2408dd3d66a4ede0ceaf783055dcb39d0..e8a8d8e1dfa0063393ca2f2c46fd609d9abedb78 100644
--- a/llvm/test/Instrumentation/AddressSanitizer/global_metadata_array.ll
+++ b/llvm/test/Instrumentation/AddressSanitizer/global_metadata_array.ll
@@ -1,9 +1,9 @@
-; RUN: opt < %s -asan -asan-module -enable-new-pm=0 -asan-globals-live-support=0 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefixes=CHECK,CHECK-S3 %s
-; RUN: opt < %s -passes='asan-pipeline' -asan-globals-live-support=0 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefixes=CHECK,CHECK-S3 %s
-; RUN: opt < %s -asan -asan-module -enable-new-pm=0 -asan-globals-live-support=0 -mtriple=x86_64-apple-macosx10.11.0 -S | FileCheck --check-prefixes=CHECK,CHECK-S3 %s
-; RUN: opt < %s -passes='asan-pipeline' -asan-globals-live-support=0 -mtriple=x86_64-apple-macosx10.11.0 -S | FileCheck --check-prefixes=CHECK,CHECK-S3 %s
-; RUN: opt < %s -asan -asan-module -enable-new-pm=0 -asan-globals-live-support=0 -mtriple=x86_64-pc-windows-msvc19.0.24215 -S | FileCheck --check-prefixes=CHECK,CHECK-S3 %s
-; RUN: opt < %s -passes='asan-pipeline' -asan-globals-live-support=0 -mtriple=x86_64-pc-windows-msvc19.0.24215 -S | FileCheck --check-prefixes=CHECK,CHECK-S3 %s
+; RUN: opt < %s -asan -asan-module -enable-new-pm=0 -asan-globals-live-support=0 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefix=CHECK %s
+; RUN: opt < %s -passes='asan-pipeline' -asan-globals-live-support=0 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefix=CHECK %s
+; RUN: opt < %s -asan -asan-module -enable-new-pm=0 -asan-globals-live-support=0 -mtriple=x86_64-apple-macosx10.11.0 -S | FileCheck --check-prefix=CHECK %s
+; RUN: opt < %s -passes='asan-pipeline' -asan-globals-live-support=0 -mtriple=x86_64-apple-macosx10.11.0 -S | FileCheck --check-prefix=CHECK %s
+; RUN: opt < %s -asan -asan-module -enable-new-pm=0 -asan-globals-live-support=0 -mtriple=x86_64-pc-windows-msvc19.0.24215 -S | FileCheck --check-prefix=CHECK %s
+; RUN: opt < %s -passes='asan-pipeline' -asan-globals-live-support=0 -mtriple=x86_64-pc-windows-msvc19.0.24215 -S | FileCheck --check-prefix=CHECK %s
 ; RUN: opt < %s -asan -asan-module -enable-new-pm=0 -asan-globals-live-support=0 -asan-mapping-scale=5 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefixes=CHECK,CHECK-S5 %s
 ; RUN: opt < %s -passes='asan-pipeline' -asan-globals-live-support=0 -asan-mapping-scale=5 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefixes=CHECK,CHECK-S5 %s
 
diff --git a/llvm/test/Instrumentation/HWAddressSanitizer/basic.ll b/llvm/test/Instrumentation/HWAddressSanitizer/basic.ll
index 130e20dfea87f00fa53cfabc3044094d67a43ac1..e139e3c9cbafd1fc64ecf0a00157371637128da9 100644
--- a/llvm/test/Instrumentation/HWAddressSanitizer/basic.ll
+++ b/llvm/test/Instrumentation/HWAddressSanitizer/basic.ll
@@ -1,14 +1,14 @@
 ; Test basic address sanitizer instrumentation.
 ;
-; RUN: opt < %s -hwasan -hwasan-recover=0 -hwasan-with-ifunc=1 -hwasan-with-tls=0 -S | FileCheck %s --check-prefixes=CHECK,ABORT,ABORT-DYNAMIC-SHADOW
+; RUN: opt < %s -hwasan -hwasan-recover=0 -hwasan-with-ifunc=1 -hwasan-with-tls=0 -S | FileCheck %s --check-prefixes=CHECK,ABORT
 ; RUN: opt < %s -hwasan -hwasan-recover=1 -hwasan-with-ifunc=1 -hwasan-with-tls=0 -S | FileCheck %s --check-prefixes=CHECK,RECOVER,RECOVER-DYNAMIC-SHADOW
-; RUN: opt < %s -hwasan -hwasan-recover=0 -hwasan-mapping-offset=0 -S | FileCheck %s --check-prefixes=CHECK,ABORT,ABORT-ZERO-BASED-SHADOW
+; RUN: opt < %s -hwasan -hwasan-recover=0 -hwasan-mapping-offset=0 -S | FileCheck %s --check-prefixes=CHECK,ABORT
 ; RUN: opt < %s -hwasan -hwasan-recover=1 -hwasan-mapping-offset=0 -S | FileCheck %s --check-prefixes=CHECK,RECOVER,RECOVER-ZERO-BASED-SHADOW
 
 ; Ensure than hwasan runs with the new PM pass
-; RUN: opt < %s -passes=hwasan -hwasan-recover=0 -hwasan-with-ifunc=1 -hwasan-with-tls=0 -S | FileCheck %s --check-prefixes=CHECK,ABORT,ABORT-DYNAMIC-SHADOW
+; RUN: opt < %s -passes=hwasan -hwasan-recover=0 -hwasan-with-ifunc=1 -hwasan-with-tls=0 -S | FileCheck %s --check-prefixes=CHECK,ABORT
 ; RUN: opt < %s -passes=hwasan -hwasan-recover=1 -hwasan-with-ifunc=1 -hwasan-with-tls=0 -S | FileCheck %s --check-prefixes=CHECK,RECOVER,RECOVER-DYNAMIC-SHADOW
-; RUN: opt < %s -passes=hwasan -hwasan-recover=0 -hwasan-mapping-offset=0 -S | FileCheck %s --check-prefixes=CHECK,ABORT,ABORT-ZERO-BASED-SHADOW
+; RUN: opt < %s -passes=hwasan -hwasan-recover=0 -hwasan-mapping-offset=0 -S | FileCheck %s --check-prefixes=CHECK,ABORT
 ; RUN: opt < %s -passes=hwasan -hwasan-recover=1 -hwasan-mapping-offset=0 -S | FileCheck %s --check-prefixes=CHECK,RECOVER,RECOVER-ZERO-BASED-SHADOW
 
 ; CHECK: @llvm.global_ctors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 0, void ()* @hwasan.module_ctor, i8* bitcast (void ()* @hwasan.module_ctor to i8*) }]
diff --git a/llvm/test/Instrumentation/HWAddressSanitizer/prologue.ll b/llvm/test/Instrumentation/HWAddressSanitizer/prologue.ll
index babfd9d725d62701d3b62b1c88a2bcb18c068d31..d74940fe68a28090bbebae9a687747cfd2b08025 100644
--- a/llvm/test/Instrumentation/HWAddressSanitizer/prologue.ll
+++ b/llvm/test/Instrumentation/HWAddressSanitizer/prologue.ll
@@ -9,7 +9,7 @@
 ; RUN: opt -hwasan -S -hwasan-with-ifunc=0 -hwasan-with-tls=0 < %s | \
 ; RUN:     FileCheck %s --check-prefixes=CHECK,CHECK-GLOBAL,CHECK-NOHISTORY
 ; RUN: opt -hwasan -S -hwasan-with-ifunc=1  -hwasan-with-tls=0 < %s | \
-; RUN:     FileCheck %s --check-prefixes=CHECK,CHECk-NOGLOBAL,CHECK-IFUNC,CHECK-NOHISTORY
+; RUN:     FileCheck %s --check-prefixes=CHECK,CHECK-IFUNC,CHECK-NOHISTORY
 
 target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
 target triple = "aarch64--linux-android22"
diff --git a/llvm/test/Instrumentation/MemorySanitizer/array_types.ll b/llvm/test/Instrumentation/MemorySanitizer/array_types.ll
index cc397e12e42c30d0df6065832b02ec580fb6d07c..0beade542dbcb8bc36ba62ad0e338f5823a30ff1 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/array_types.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/array_types.ll
@@ -3,8 +3,8 @@
 ; RUN: opt < %s -msan -msan-check-access-address=0 -S | FileCheck %s
 ; RUN: opt < %s -msan-check-access-address=0 -msan-track-origins=1 -S          \
 ; RUN: -passes=msan 2>&1 | FileCheck -check-prefix=CHECK                       \
-; RUN: -check-prefix=CHECK-ORIGINS %s --allow-empty
-; RUN: opt < %s -msan -msan-check-access-address=0 -msan-track-origins=1 -S | FileCheck -check-prefix=CHECK -check-prefix=CHECK-ORIGINS %s
+; RUN: %s --allow-empty
+; RUN: opt < %s -msan -msan-check-access-address=0 -msan-track-origins=1 -S | FileCheck -check-prefix=CHECK %s
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/llvm/test/Instrumentation/MemorySanitizer/check-array.ll b/llvm/test/Instrumentation/MemorySanitizer/check-array.ll
index 6d1e517a186d48a6ac6f6abbc5d2dfc7b9e88633..283913fc658a9a9ef4f22f429c14710ad5be6b97 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/check-array.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/check-array.ll
@@ -1,5 +1,5 @@
 ; RUN: opt < %s -msan-eager-checks -msan-check-access-address=0 -msan-track-origins=1 -S -passes='module(msan-module),function(msan)' 2>&1 | \
-; RUN:   FileCheck -allow-deprecated-dag-overlap -check-prefixes=CHECK,CHECK-ORIGINS %s
+; RUN:   FileCheck -allow-deprecated-dag-overlap --check-prefix=CHECK %s
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/llvm/test/Instrumentation/MemorySanitizer/check-struct.ll b/llvm/test/Instrumentation/MemorySanitizer/check-struct.ll
index 344dab45db231988eff4f79ef038afc1509fa21c..1ecf3fc9a6039e68ed55164c97ee9527c79a916b 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/check-struct.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/check-struct.ll
@@ -1,5 +1,5 @@
 ; RUN: opt < %s -msan-check-access-address=0 -msan-track-origins=1 -S -passes='module(msan-module),function(msan)' 2>&1 | \
-; RUN:   FileCheck -allow-deprecated-dag-overlap -check-prefixes=CHECK,CHECK-ORIGINS %s
+; RUN:   FileCheck -allow-deprecated-dag-overlap --check-prefix=CHECK %s
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/llvm/test/Instrumentation/MemorySanitizer/freeze.ll b/llvm/test/Instrumentation/MemorySanitizer/freeze.ll
index 4be8d9efd63195d0e10d1fb0f0e57d6e21a4b985..32cb5b77cee270862ce8d289030eccf70dfcd54b 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/freeze.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/freeze.ll
@@ -1,5 +1,5 @@
 ; RUN: opt < %s -msan-check-access-address=0 -S -passes=msan 2>&1 | FileCheck %s
-; RUN: opt < %s -msan-check-access-address=0 -msan-track-origins=2 -S -passes=msan 2>&1 | FileCheck %s -check-prefixes=CHECK,CHECK-ORIGIN
+; RUN: opt < %s -msan-check-access-address=0 -msan-track-origins=2 -S -passes=msan 2>&1 | FileCheck %s
 ; RUN: opt < %s -msan -msan-check-access-address=0 -S | FileCheck %s
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/llvm/test/Instrumentation/MemorySanitizer/msan_asm_conservative.ll b/llvm/test/Instrumentation/MemorySanitizer/msan_asm_conservative.ll
index 296e62a9200e5fe11a1f87216adc8655d9bf4f9f..749145142445b607b6e8fada2b06c689be537665 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/msan_asm_conservative.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/msan_asm_conservative.ll
@@ -1,8 +1,8 @@
 ; Test for handling of asm constraints in MSan instrumentation.
 ; RUN: opt < %s -msan-kernel=1 -msan-check-access-address=0                    \
 ; RUN: -msan-handle-asm-conservative=0 -S -passes=msan 2>&1 | FileCheck        \
-; RUN: "-check-prefixes=CHECK,CHECK-NONCONS" %s
-; RUN: opt < %s -msan -msan-kernel=1 -msan-check-access-address=0 -msan-handle-asm-conservative=0 -S | FileCheck -check-prefixes=CHECK,CHECK-NONCONS %s
+; RUN: "-check-prefix=CHECK" %s
+; RUN: opt < %s -msan -msan-kernel=1 -msan-check-access-address=0 -msan-handle-asm-conservative=0 -S | FileCheck -check-prefixes=CHECK %s
 ; RUN: opt < %s -msan-kernel=1 -msan-check-access-address=0                    \
 ; RUN: -msan-handle-asm-conservative=1 -S -passes=msan 2>&1 | FileCheck        \
 ; RUN: "-check-prefixes=CHECK,CHECK-CONS" %s
diff --git a/llvm/test/Instrumentation/MemorySanitizer/msan_eager.ll b/llvm/test/Instrumentation/MemorySanitizer/msan_eager.ll
index bb44dea03da9a37a4a24279619e9577a467c7a23..3912b9ad1d90d78c5b5708572cba6721d2dfdbb4 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/msan_eager.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/msan_eager.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -msan-check-access-address=0 -msan-track-origins=1 -msan-eager-checks -S -passes='module(msan-module),function(msan)' 2>&1 | \
-; RUN:   FileCheck -allow-deprecated-dag-overlap -check-prefixes=CHECK,CHECK-ORIGINS %s
+; RUN:   FileCheck -allow-deprecated-dag-overlap --check-prefix=CHECK %s
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/llvm/test/Instrumentation/MemorySanitizer/msan_x86_bts_asm.ll b/llvm/test/Instrumentation/MemorySanitizer/msan_x86_bts_asm.ll
index d9bf307c2b525bf10e5a446f75bfc53771453abb..22c206faa0e96a56d319915fa4876ff309116ab1 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/msan_x86_bts_asm.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/msan_x86_bts_asm.ll
@@ -1,8 +1,8 @@
 ; Test for the conservative assembly handling mode used by KMSAN.
 ; RUN: opt < %s -msan-kernel=1 -msan-check-access-address=0                    \
 ; RUN: -msan-handle-asm-conservative=0 -S -passes=msan 2>&1 | FileCheck        \
-; RUN: "-check-prefixes=CHECK,CHECK-NONCONS" %s
-; RUN: opt < %s -msan -msan-kernel=1 -msan-check-access-address=0 -msan-handle-asm-conservative=0 -S | FileCheck -check-prefixes=CHECK,CHECK-NONCONS %s
+; RUN: "-check-prefix=CHECK" %s
+; RUN: opt < %s -msan -msan-kernel=1 -msan-check-access-address=0 -msan-handle-asm-conservative=0 -S | FileCheck -check-prefixes=CHECK %s
 ; RUN: opt < %s -msan-kernel=1 -msan-check-access-address=0                    \
 ; RUN: -msan-handle-asm-conservative=1 -S -passes=msan 2>&1 | FileCheck        \
 ; RUN: "-check-prefixes=CHECK,CHECK-CONS" %s
diff --git a/llvm/test/Instrumentation/MemorySanitizer/reduce.ll b/llvm/test/Instrumentation/MemorySanitizer/reduce.ll
index 2629756644e59401ecbb7248fefe00ff85e28f90..add81a5d883a0d5b3cda3448f7db981cc770c2ff 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/reduce.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/reduce.ll
@@ -1,6 +1,6 @@
 
 ; RUN: opt < %s -msan-check-access-address=0 -msan-track-origins=1 -S -passes='module(msan-module),function(msan)' 2>&1 | \
-; RUN:   FileCheck -allow-deprecated-dag-overlap -check-prefixes=CHECK,CHECK-ORIGINS %s
+; RUN:   FileCheck -allow-deprecated-dag-overlap --check-prefix=CHECK %s
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/llvm/test/MC/AArch64/lit.local.cfg b/llvm/test/MC/AArch64/lit.local.cfg
index ab829130e2696acc6764f7f9eb12b99586902b2b..5822b72266874f012f161d6aaef5962644d37c4b 100644
--- a/llvm/test/MC/AArch64/lit.local.cfg
+++ b/llvm/test/MC/AArch64/lit.local.cfg
@@ -1,10 +1,2 @@
-from lit.llvm.subst import ToolSubst
-
 if 'AArch64' not in config.root.targets:
     config.unsupported = True
-
-fc = ToolSubst('FileCheck', unresolved='fatal')
-# Insert this first. Then, we'll first update the blank FileCheck command; then,
-# the default substitution of FileCheck will replace it to its full path.
-config.substitutions.insert(0, (fc.regex,
-    'FileCheck --allow-unused-prefixes=false'))
diff --git a/llvm/test/MC/AMDGPU/lit.local.cfg b/llvm/test/MC/AMDGPU/lit.local.cfg
index 27c8bcd8e82f29f0f24a4e908abb781070f5d86c..2a665f06be72e5515ca6e27018facb35daa201be 100644
--- a/llvm/test/MC/AMDGPU/lit.local.cfg
+++ b/llvm/test/MC/AMDGPU/lit.local.cfg
@@ -1,10 +1,2 @@
-from lit.llvm.subst import ToolSubst
-
 if not 'AMDGPU' in config.root.targets:
     config.unsupported = True
-
-fc = ToolSubst('FileCheck', unresolved='fatal')
-# Insert this first. Then, we'll first update the blank FileCheck command; then,
-# the default substitution of FileCheck will replace it to its full path.
-config.substitutions.insert(0, (fc.regex,
-    'FileCheck --allow-unused-prefixes=false'))
diff --git a/llvm/test/MC/ARM/lit.local.cfg b/llvm/test/MC/ARM/lit.local.cfg
index b305cf706e7781ff25652191484e92cf4e2e8472..236e1d34416659068d4434bd878989a4165d35c4 100644
--- a/llvm/test/MC/ARM/lit.local.cfg
+++ b/llvm/test/MC/ARM/lit.local.cfg
@@ -1,10 +1,2 @@
-from lit.llvm.subst import ToolSubst
-
 if not 'ARM' in config.root.targets:
     config.unsupported = True
-
-fc = ToolSubst('FileCheck', unresolved='fatal')
-# Insert this first. Then, we'll first update the blank FileCheck command; then,
-# the default substitution of FileCheck will replace it to its full path.
-config.substitutions.insert(0, (fc.regex,
-    'FileCheck --allow-unused-prefixes=false'))
diff --git a/llvm/test/MC/RISCV/lit.local.cfg b/llvm/test/MC/RISCV/lit.local.cfg
index 896e7941295e36ce6b05cb9311087dff3631801d..c63820126f8ca6ce0511f50de23b37b5e484bb70 100644
--- a/llvm/test/MC/RISCV/lit.local.cfg
+++ b/llvm/test/MC/RISCV/lit.local.cfg
@@ -1,10 +1,2 @@
-from lit.llvm.subst import ToolSubst
-
 if not 'RISCV' in config.root.targets:
     config.unsupported = True
-
-fc = ToolSubst('FileCheck', unresolved='fatal')
-# Insert this first. Then, we'll first update the blank FileCheck command; then,
-# the default substitution of FileCheck will replace it to its full path.
-config.substitutions.insert(0, (fc.regex,
-    'FileCheck --allow-unused-prefixes=false'))
diff --git a/llvm/test/Other/lit.local.cfg b/llvm/test/Other/lit.local.cfg
deleted file mode 100644
index 1b775f32afe07e6e4850bed15ff79a5a6febbc65..0000000000000000000000000000000000000000
--- a/llvm/test/Other/lit.local.cfg
+++ /dev/null
@@ -1,12 +0,0 @@
-from lit.llvm.subst import ToolSubst
-
-fc = ToolSubst('FileCheck', unresolved='fatal')
-# Insert this first. Then, we'll first update the blank FileCheck command; then,
-# the default substitution of FileCheck will replace it to its full path.
-config.substitutions.insert(0, (fc.regex,
-    'FileCheck --allow-unused-prefixes=false'))
-
-# FIXME: this substitution should be removed together with the above, when the
-# default --allow-unused-prefixes is fixed. At that point, the use of %FileCheckRaw%
-# in opt-bisect-legacy-pass-manager.ll should be switched back to just FileCheck.
-config.substitutions.append(('%FileCheckRaw%', 'FileCheck'))
diff --git a/llvm/test/Reduce/lit.local.cfg b/llvm/test/Reduce/lit.local.cfg
new file mode 100644
index 0000000000000000000000000000000000000000..dcd414b5eaf507faf104ad0752e46ee42478070e
--- /dev/null
+++ b/llvm/test/Reduce/lit.local.cfg
@@ -0,0 +1,8 @@
+# -*- Python -*- vim: set ft=python ts=4 sw=4 expandtab tw=79:
+# FIXME: remove this file when we flip the default for --allow-unused-prefixes.
+from lit.llvm.subst import ToolSubst
+
+fc = ToolSubst('FileCheck', unresolved='fatal')
+# the parent introduced the opposite rule, so we replace it if we see it.
+if len(config.substitutions) > 0 and config.substitutions[0] == (fc.regex, 'FileCheck --allow-unused-prefixes=false'):
+    del config.substitutions[0]
diff --git a/llvm/test/Transforms/lit.local.cfg b/llvm/test/Transforms/lit.local.cfg
deleted file mode 100644
index 0a317f9a9ff90d9fe9716efe51cc21a0dbfad374..0000000000000000000000000000000000000000
--- a/llvm/test/Transforms/lit.local.cfg
+++ /dev/null
@@ -1,8 +0,0 @@
-# -*- Python -*- vim: set ft=python ts=4 sw=4 expandtab tw=79:
-from lit.llvm.subst import ToolSubst
-
-fc = ToolSubst('FileCheck', unresolved='fatal')
-# Insert this first. Then, we'll first update the blank FileCheck command; then,
-# the default substitution of FileCheck will replace it to its full path.
-config.substitutions.insert(0, (fc.regex, 
-    'FileCheck --allow-unused-prefixes=false'))
diff --git a/llvm/test/lit.cfg.py b/llvm/test/lit.cfg.py
index 2fb5d021d0fc58d85ec3d3523f6eb73450e8f2dc..348ab62c7bb89eb987f9d38d10c34feb08cad3ee 100644
--- a/llvm/test/lit.cfg.py
+++ b/llvm/test/lit.cfg.py
@@ -82,10 +82,20 @@ def get_asan_rtlib():
         return ''
     return found_dylibs[0]
 
-# Define this first. Afterwards, use_default_substitutions will add the rule for
-# expanding FileCheck to the full path.
-config.substitutions.append(('%FileCheckWithUnusedPrefixes%', 
-    'FileCheck --allow-unused-prefixes=true'))
+
+####################################################
+# FIXME: remove this when we flip the default value for --allow-unused-prefixes
+# to false.
+fc = ToolSubst('FileCheck', unresolved='fatal')
+# Insert this first. Then, we'll first update the blank FileCheck command; then,
+# the default substitution of FileCheck will replace it to its full path.
+config.substitutions.insert(0, (fc.regex,
+    'FileCheck --allow-unused-prefixes=false'))
+# When addressing this fixme, replace %FileCheckRaw% with just FileCheck.
+config.substitutions.append(('%FileCheckRaw%', 'FileCheck'))
+# Also remove the lit.local.cfg under llvm/test/Reduce
+# and the pertinent FIXME in llvm/test/FileCheck
+####################################################
 
 llvm_config.use_default_substitutions()