diff --git a/llvm/test/CodeGen/X86/fptosi-sat-scalar.ll b/llvm/test/CodeGen/X86/fptosi-sat-scalar.ll
--- a/llvm/test/CodeGen/X86/fptosi-sat-scalar.ll
+++ b/llvm/test/CodeGen/X86/fptosi-sat-scalar.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc < %s -mtriple=i686-linux | FileCheck %s --check-prefixes=X86,X86-X87
-; RUN: llc < %s -mtriple=i686-linux -mattr=+sse2 | FileCheck %s --check-prefixes=X86,X86-SSE
+; RUN: llc < %s -mtriple=i686-linux | FileCheck %s --check-prefix=X86-X87
+; RUN: llc < %s -mtriple=i686-linux -mattr=+sse2 | FileCheck %s --check-prefix=X86-SSE
 ; RUN: llc < %s -mtriple=x86_64-linux | FileCheck %s --check-prefix=X64
 
 ;
diff --git a/llvm/test/CodeGen/X86/fptoui-sat-scalar.ll b/llvm/test/CodeGen/X86/fptoui-sat-scalar.ll
--- a/llvm/test/CodeGen/X86/fptoui-sat-scalar.ll
+++ b/llvm/test/CodeGen/X86/fptoui-sat-scalar.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc < %s -mtriple=i686-linux | FileCheck %s --check-prefixes=X86,X86-X87
-; RUN: llc < %s -mtriple=i686-linux -mattr=+sse2 | FileCheck %s --check-prefixes=X86,X86-SSE
+; RUN: llc < %s -mtriple=i686-linux | FileCheck %s --check-prefix=X86-X87
+; RUN: llc < %s -mtriple=i686-linux -mattr=+sse2 | FileCheck %s --check-prefix=X86-SSE
 ; RUN: llc < %s -mtriple=x86_64-linux | FileCheck %s --check-prefix=X64
 
 ;
diff --git a/llvm/test/CodeGen/X86/lit.local.cfg b/llvm/test/CodeGen/X86/lit.local.cfg
--- a/llvm/test/CodeGen/X86/lit.local.cfg
+++ b/llvm/test/CodeGen/X86/lit.local.cfg
@@ -1,2 +1,10 @@
+from lit.llvm.subst import ToolSubst
+
 if not 'X86' 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/CodeGen/X86/vector-pack-128.ll b/llvm/test/CodeGen/X86/vector-pack-128.ll
--- a/llvm/test/CodeGen/X86/vector-pack-128.ll
+++ b/llvm/test/CodeGen/X86/vector-pack-128.ll
@@ -3,8 +3,8 @@
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+sse4.2 | FileCheck %s --check-prefixes=SSE,SSE4
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx    | FileCheck %s --check-prefixes=AVX,AVX1
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx2   | FileCheck %s --check-prefixes=AVX,AVX2
-; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx512vl,+avx512f  | FileCheck %s --check-prefixes=AVX,AVX512,AVX512F
-; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx512vl,+avx512bw | FileCheck %s --check-prefixes=AVX,AVX512,AVX512BW
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx512vl,+avx512f  | FileCheck %s --check-prefixes=AVX,AVX512
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx512vl,+avx512bw | FileCheck %s --check-prefixes=AVX,AVX512
 
 ; trunc(concat(x,y)) -> pack