diff --git a/llvm/test/Transforms/ArgumentPromotion/crash.ll b/llvm/test/Transforms/ArgumentPromotion/crash.ll
--- a/llvm/test/Transforms/ArgumentPromotion/crash.ll
+++ b/llvm/test/Transforms/ArgumentPromotion/crash.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --function-signature --scrub-attributes
-; RUN: opt -S < %s -inline -argpromotion | FileCheck %s --check-prefixes=ARGPROMOTION,ALL_OLDPM
+; RUN: opt -S < %s -inline -argpromotion | FileCheck %s --check-prefix=ARGPROMOTION
 ; RUN: opt -S < %s -passes=inline,argpromotion | FileCheck %s --check-prefixes=ARGPROMOTION,ALL_NEWPM
 
 %S = type { %S* }
diff --git a/llvm/test/Transforms/Attributor/lit.local.cfg b/llvm/test/Transforms/Attributor/lit.local.cfg
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/Attributor/lit.local.cfg
@@ -0,0 +1,11 @@
+# -*- Python -*- vim: set ft=python ts=4 sw=4 expandtab tw=79:
+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'):
+    config.substitutions[0] = (
+        fc.regex, 'FileCheck --allow-unused-prefixes=true')
+else:
+    config.substitutions.insert(0, (fc.regex, 
+                                    'FileCheck --allow-unused-prefixes=true'))
diff --git a/llvm/test/Transforms/HardwareLoops/scalar-while.ll b/llvm/test/Transforms/HardwareLoops/scalar-while.ll
--- a/llvm/test/Transforms/HardwareLoops/scalar-while.ll
+++ b/llvm/test/Transforms/HardwareLoops/scalar-while.ll
@@ -1,9 +1,9 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -S %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-DEC
-; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -force-hardware-loop-phi=true -S %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-PHI
-; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -force-nested-hardware-loop=true -S %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-NESTED
-; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -force-hardware-loop-guard=true -S %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-GUARD
-; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -force-hardware-loop-phi=true -force-hardware-loop-guard=true -S %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-PHIGUARD
+; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -S %s -o - | FileCheck %s --check-prefix=CHECK-DEC
+; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -force-hardware-loop-phi=true -S %s -o - | FileCheck %s --check-prefix=CHECK-PHI
+; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -force-nested-hardware-loop=true -S %s -o - | FileCheck %s --check-prefix=CHECK-NESTED
+; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -force-hardware-loop-guard=true -S %s -o - | FileCheck %s --check-prefix=CHECK-GUARD
+; RUN: opt -hardware-loops -force-hardware-loops=true -hardware-loop-decrement=1 -hardware-loop-counter-bitwidth=32 -force-hardware-loop-phi=true -force-hardware-loop-guard=true -S %s -o - | FileCheck %s --check-prefix=CHECK-PHIGUARD
 
 define void @while_lt(i32 %i, i32 %N, i32* nocapture %A) {
 ; CHECK-DEC-LABEL: @while_lt(
diff --git a/llvm/test/Transforms/Inline/inline_stats.ll b/llvm/test/Transforms/Inline/inline_stats.ll
--- a/llvm/test/Transforms/Inline/inline_stats.ll
+++ b/llvm/test/Transforms/Inline/inline_stats.ll
@@ -1,16 +1,16 @@
 ; First with legacy PM
-; RUN: opt -S -inline -inliner-function-import-stats=basic < %s 2>&1 | FileCheck %s -check-prefix=CHECK-BASIC -check-prefix=CHECK
-; RUN: opt -S -inline -inliner-function-import-stats=verbose < %s 2>&1 | FileCheck %s -check-prefix="CHECK-VERBOSE" -check-prefix=CHECK
+; RUN: opt -S -inline -inliner-function-import-stats=basic < %s 2>&1 | FileCheck %s --check-prefixes=CHECK-BASIC,CHECK
+; RUN: opt -S -inline -inliner-function-import-stats=verbose < %s 2>&1 | FileCheck %s --check-prefixes="CHECK-VERBOSE",CHECK
 
 ; Do again with new PM
-; RUN: opt -S -passes=inline -inliner-function-import-stats=basic < %s 2>&1 | FileCheck %s -check-prefix=CHECK-BASIC -check-prefix=CHECK
-; RUN: opt -S -passes=inline -inliner-function-import-stats=verbose < %s 2>&1 | FileCheck %s -check-prefix="CHECK-VERBOSE" -check-prefix=CHECK
+; RUN: opt -S -passes=inline -inliner-function-import-stats=basic < %s 2>&1 | FileCheck %s --check-prefixes=CHECK-BASIC,CHECK
+; RUN: opt -S -passes=inline -inliner-function-import-stats=verbose < %s 2>&1 | FileCheck %s --check-prefixes="CHECK-VERBOSE",CHECK
 
-; RUN: opt -S -passes=inliner-wrapper -inliner-function-import-stats=basic < %s 2>&1 | FileCheck %s -check-prefix=CHECK-BASIC -check-prefix=CHECK
-; RUN: opt -S -passes=inliner-wrapper -inliner-function-import-stats=verbose < %s 2>&1 | FileCheck %s -check-prefix="CHECK-VERBOSE" -check-prefix=CHECK
+; RUN: opt -S -passes=inliner-wrapper -inliner-function-import-stats=basic < %s 2>&1 | FileCheck %s --check-prefixes=CHECK-BASIC,CHECK
+; RUN: opt -S -passes=inliner-wrapper -inliner-function-import-stats=verbose < %s 2>&1 | FileCheck %s --check-prefixes="CHECK-VERBOSE",CHECK
 
-; RUN: opt -S -passes=always-inliner-wrapper,inliner-wrapper -inliner-function-import-stats=basic < %s 2>&1 | FileCheck %s -check-prefix=WRAPPER-BASIC -check-prefix=WRAPPER
-; RUN: opt -S -passes=always-inliner-wrapper,inliner-wrapper -inliner-function-import-stats=verbose < %s 2>&1 | FileCheck %s -check-prefix=WRAPPER-VERBOSE -check-prefix=WRAPPER
+; RUN: opt -S -passes=always-inliner-wrapper,inliner-wrapper -inliner-function-import-stats=basic < %s 2>&1 | FileCheck %s -check-prefix=WRAPPER
+; RUN: opt -S -passes=always-inliner-wrapper,inliner-wrapper -inliner-function-import-stats=verbose < %s 2>&1 | FileCheck %s --check-prefixes=WRAPPER-VERBOSE,WRAPPER
 
 ; CHECK: ------- Dumping inliner stats for [<stdin>] -------
 ; CHECK-BASIC-NOT: -- List of inlined functions:
diff --git a/llvm/test/Transforms/Inline/noalias2.ll b/llvm/test/Transforms/Inline/noalias2.ll
--- a/llvm/test/Transforms/Inline/noalias2.ll
+++ b/llvm/test/Transforms/Inline/noalias2.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --function-signature
 ; RUN: opt -inline -enable-noalias-to-md-conversion -S < %s | FileCheck %s --check-prefixes=CHECK,NO_ASSUME
-; RUN: opt -inline -enable-noalias-to-md-conversion --enable-knowledge-retention -S < %s | FileCheck %s --check-prefixes=CHECK,USE_ASSUME
+; RUN: opt -inline -enable-noalias-to-md-conversion --enable-knowledge-retention -S < %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/Transforms/LICM/pr23608.ll b/llvm/test/Transforms/LICM/pr23608.ll
--- a/llvm/test/Transforms/LICM/pr23608.ll
+++ b/llvm/test/Transforms/LICM/pr23608.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -licm %s | FileCheck %s --check-prefixes=CHECK,NO_ASSUME
-; RUN: opt -S -licm --enable-knowledge-retention %s | FileCheck %s --check-prefixes=CHECK,USE_ASSUME
+; RUN: opt -S -licm %s | FileCheck %s --check-prefix=NO_ASSUME
+; RUN: opt -S -licm --enable-knowledge-retention %s | FileCheck %s --check-prefix=USE_ASSUME
 ; ModuleID = '../pr23608.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
diff --git a/llvm/test/Transforms/LoadStoreVectorizer/AMDGPU/merge-stores-private.ll b/llvm/test/Transforms/LoadStoreVectorizer/AMDGPU/merge-stores-private.ll
--- a/llvm/test/Transforms/LoadStoreVectorizer/AMDGPU/merge-stores-private.ll
+++ b/llvm/test/Transforms/LoadStoreVectorizer/AMDGPU/merge-stores-private.ll
@@ -1,6 +1,6 @@
 ; RUN: opt -mtriple=amdgcn-amd-amdhsa -mattr=+max-private-element-size-4,-unaligned-scratch-access  -load-store-vectorizer -S -o - %s | FileCheck -check-prefixes=ELT4,ELT4-ALIGNED,ALIGNED,ALL %s
-; RUN: opt -mtriple=amdgcn-amd-amdhsa -mattr=+max-private-element-size-8,-unaligned-scratch-access  -load-store-vectorizer -S -o - %s | FileCheck -check-prefixes=ELT8,ELT8-ALIGNED,ALIGNED,ALL %s
-; RUN: opt -mtriple=amdgcn-amd-amdhsa -mattr=+max-private-element-size-16,-unaligned-scratch-access -load-store-vectorizer -S -o - %s | FileCheck -check-prefixes=ELT16,ELT16-ALIGNED,ALIGNED,ALL %s
+; RUN: opt -mtriple=amdgcn-amd-amdhsa -mattr=+max-private-element-size-8,-unaligned-scratch-access  -load-store-vectorizer -S -o - %s | FileCheck -check-prefixes=ELT8,ALIGNED,ALL %s
+; RUN: opt -mtriple=amdgcn-amd-amdhsa -mattr=+max-private-element-size-16,-unaligned-scratch-access -load-store-vectorizer -S -o - %s | FileCheck -check-prefixes=ELT16,ALIGNED,ALL %s
 ; RUN: opt -mtriple=amdgcn-amd-amdhsa -mattr=+max-private-element-size-4,+unaligned-scratch-access  -load-store-vectorizer -S -o - %s | FileCheck -check-prefixes=ELT4,ELT4-UNALIGNED,UNALIGNED,ALL %s
 ; RUN: opt -mtriple=amdgcn-amd-amdhsa -mattr=+max-private-element-size-8,+unaligned-scratch-access  -load-store-vectorizer -S -o - %s | FileCheck -check-prefixes=ELT8,ELT8-UNALIGNED,UNALIGNED,ALL %s
 ; RUN: opt -mtriple=amdgcn-amd-amdhsa -mattr=+max-private-element-size-16,+unaligned-scratch-access -load-store-vectorizer -S -o - %s | FileCheck -check-prefixes=ELT16,ELT16-UNALIGNED,UNALIGNED,ALL %s
diff --git a/llvm/test/Transforms/LoopIdiom/X86/left-shift-until-bittest.ll b/llvm/test/Transforms/LoopIdiom/X86/left-shift-until-bittest.ll
--- a/llvm/test/Transforms/LoopIdiom/X86/left-shift-until-bittest.ll
+++ b/llvm/test/Transforms/LoopIdiom/X86/left-shift-until-bittest.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -debugify -loop-idiom -mtriple=x86_64 -mcpu=core-avx2 < %s -S | FileCheck -check-prefixes=ALL,LZCNT %s
-; RUN: opt -debugify -loop-idiom -mtriple=x86_64 -mcpu=corei7 < %s -S | FileCheck -check-prefixes=ALL,NOLZCNT %s
+; RUN: opt -debugify -loop-idiom -mtriple=x86_64 -mcpu=core-avx2 < %s -S | FileCheck --check-prefix=ALL %s
+; RUN: opt -debugify -loop-idiom -mtriple=x86_64 -mcpu=corei7 < %s -S | FileCheck --check-prefix=ALL %s
 
 declare i32 @gen32()
 declare void @use32(i32)
diff --git a/llvm/test/Transforms/LoopStrengthReduce/X86/lsr-insns-1.ll b/llvm/test/Transforms/LoopStrengthReduce/X86/lsr-insns-1.ll
--- a/llvm/test/Transforms/LoopStrengthReduce/X86/lsr-insns-1.ll
+++ b/llvm/test/Transforms/LoopStrengthReduce/X86/lsr-insns-1.ll
@@ -1,7 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -loop-reduce -mtriple=x86_64  -S | FileCheck %s -check-prefix=BOTH -check-prefix=INSN
-; RUN: opt < %s -loop-reduce -mtriple=x86_64 -lsr-insns-cost=false -S | FileCheck %s -check-prefix=BOTH -check-prefix=REGS
+; RUN: opt < %s -loop-reduce -mtriple=x86_64  -S | FileCheck %s -check-prefix=INSN
+; RUN: opt < %s -loop-reduce -mtriple=x86_64 -lsr-insns-cost=false -S | FileCheck %s -check-prefix=REGS
 ; RUN: llc < %s -O2 -mtriple=x86_64-unknown-unknown -lsr-insns-cost | FileCheck %s
 
 ; OPT test checks that LSR optimize compare for static counter to compare with 0.
diff --git a/llvm/test/Transforms/LowerTypeTests/export-inline.ll b/llvm/test/Transforms/LowerTypeTests/export-inline.ll
--- a/llvm/test/Transforms/LowerTypeTests/export-inline.ll
+++ b/llvm/test/Transforms/LowerTypeTests/export-inline.ll
@@ -1,7 +1,7 @@
-; RUN: opt -mtriple=x86_64-unknown-linux -S -lowertypetests -lowertypetests-summary-action=export -lowertypetests-read-summary=%S/Inputs/use-typeid1-typeid2.yaml -lowertypetests-write-summary=%t < %s | FileCheck --check-prefixes=CHECK,X86 %s
+; RUN: opt -mtriple=x86_64-unknown-linux -S -lowertypetests -lowertypetests-summary-action=export -lowertypetests-read-summary=%S/Inputs/use-typeid1-typeid2.yaml -lowertypetests-write-summary=%t < %s | FileCheck --check-prefix=CHECK %s
 ; RUN: FileCheck --check-prefixes=SUMMARY,SUMMARY-X86 %s < %t
 
-; RUN: opt -mtriple=aarch64-unknown-linux -S -lowertypetests -lowertypetests-summary-action=export -lowertypetests-read-summary=%S/Inputs/use-typeid1-typeid2.yaml -lowertypetests-write-summary=%t < %s | FileCheck --check-prefixes=CHECK,ARM %s
+; RUN: opt -mtriple=aarch64-unknown-linux -S -lowertypetests -lowertypetests-summary-action=export -lowertypetests-read-summary=%S/Inputs/use-typeid1-typeid2.yaml -lowertypetests-write-summary=%t < %s | FileCheck --check-prefix=CHECK %s
 ; RUN: FileCheck --check-prefixes=SUMMARY,SUMMARY-ARM %s < %t
 
 @foo = constant [2048 x i8] zeroinitializer, !type !0, !type !1, !type !2, !type !3
diff --git a/llvm/test/Transforms/MemCpyOpt/memcpy-invoke-memcpy.ll b/llvm/test/Transforms/MemCpyOpt/memcpy-invoke-memcpy.ll
--- a/llvm/test/Transforms/MemCpyOpt/memcpy-invoke-memcpy.ll
+++ b/llvm/test/Transforms/MemCpyOpt/memcpy-invoke-memcpy.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -memcpyopt -S -enable-memcpyopt-memoryssa=0 | FileCheck %s --check-prefixes=CHECK,NO_MSSA
-; RUN: opt < %s -memcpyopt -S -enable-memcpyopt-memoryssa=1 -verify-memoryssa | FileCheck %s --check-prefixes=CHECK,MSSA
+; RUN: opt < %s -memcpyopt -S -enable-memcpyopt-memoryssa=0 | FileCheck %s --check-prefix=NO_MSSA
+; RUN: opt < %s -memcpyopt -S -enable-memcpyopt-memoryssa=1 -verify-memoryssa | FileCheck %s --check-prefix=MSSA
 
 ; Test memcpy-memcpy dependencies across invoke edges.
 
diff --git a/llvm/test/Transforms/PhaseOrdering/ARM/arm_fill_q7.ll b/llvm/test/Transforms/PhaseOrdering/ARM/arm_fill_q7.ll
--- a/llvm/test/Transforms/PhaseOrdering/ARM/arm_fill_q7.ll
+++ b/llvm/test/Transforms/PhaseOrdering/ARM/arm_fill_q7.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -O3 -S                                        | FileCheck -check-prefixes=ALL,OLDPM %s
-; RUN: opt < %s -passes='default<O3>' -aa-pipeline=default -S | FileCheck -check-prefixes=ALL,NEWPM %s
+; RUN: opt < %s -O3 -S                                        | FileCheck --check-prefix=OLDPM %s
+; RUN: opt < %s -passes='default<O3>' -aa-pipeline=default -S | FileCheck --check-prefix=NEWPM %s
 
 target datalayout = "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
 target triple = "thumbv6m-none-none-eabi"
diff --git a/llvm/test/Transforms/PhaseOrdering/X86/loop-idiom-vs-indvars.ll b/llvm/test/Transforms/PhaseOrdering/X86/loop-idiom-vs-indvars.ll
--- a/llvm/test/Transforms/PhaseOrdering/X86/loop-idiom-vs-indvars.ll
+++ b/llvm/test/Transforms/PhaseOrdering/X86/loop-idiom-vs-indvars.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -mcpu=core-avx2 < %s -O3 -S                                        | FileCheck -check-prefixes=ALL,OLDPM %s
-; RUN: opt -mcpu=core-avx2 < %s -passes='default<O3>' -aa-pipeline=default -S | FileCheck -check-prefixes=ALL,NEWPM %s
+; RUN: opt -mcpu=core-avx2 < %s -O3 -S                                        | FileCheck --check-prefix=ALL %s
+; RUN: opt -mcpu=core-avx2 < %s -passes='default<O3>' -aa-pipeline=default -S | FileCheck --check-prefix=ALL %s
 
 ; Not only should we be able to make the loop countable,
 ; %whatever.next recurrence should be rewritten, making loop dead.
diff --git a/llvm/test/Transforms/lit.local.cfg b/llvm/test/Transforms/lit.local.cfg
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/lit.local.cfg
@@ -0,0 +1,8 @@
+# -*- 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
--- a/llvm/test/lit.cfg.py
+++ b/llvm/test/lit.cfg.py
@@ -82,6 +82,10 @@
         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'))
 
 llvm_config.use_default_substitutions()