diff --git a/llvm/docs/TestingGuide.rst b/llvm/docs/TestingGuide.rst
--- a/llvm/docs/TestingGuide.rst
+++ b/llvm/docs/TestingGuide.rst
@@ -612,6 +612,13 @@
 
    Example: ``Windows %errc_ENOENT: no such file or directory``
 
+``%if feature {<if branch>} %else {<else branch>}``
+
+ Conditional substitution: if ``feature`` is available it expands to
+ ``<if branch>``, otherwise it expands to ``<else branch>``.
+ ``%else {<else branch>}`` is optional and treated like ``%else {}``
+ if not present.
+
 **LLVM-specific substitutions:**
 
 ``%shlibext``
diff --git a/llvm/utils/lit/lit/TestRunner.py b/llvm/utils/lit/lit/TestRunner.py
--- a/llvm/utils/lit/lit/TestRunner.py
+++ b/llvm/utils/lit/lit/TestRunner.py
@@ -1179,7 +1179,7 @@
 def _caching_re_compile(r):
     return re.compile(r)
 
-def applySubstitutions(script, substitutions, recursion_limit=None):
+def applySubstitutions(script, substitutions, conditions={}, recursion_limit=None):
     """
     Apply substitutions to the script.  Allow full regular expression syntax.
     Replace each matching occurrence of regular expression pattern a with
@@ -1213,6 +1213,18 @@
             # seems reasonable.
             ln = _caching_re_compile(a).sub(str(b), escape(ln))
 
+        if_regex = r'%if\s+([\w\-_]+)\s+{(.*?)}(?:\s+%else\s+{(.*?)})?'
+        if_match = _caching_re_compile(if_regex).search(ln)
+        if if_match:
+            cond = if_match.group(1)
+            branch_if = if_match.group(2)
+            branch_else = if_match.group(3)
+            if conditions.get(cond, False):
+                result = branch_if or ''
+            else:
+                result = branch_else or ''
+            ln = ln[:if_match.start()] + result + ln[if_match.end():]
+
         # Strip the trailing newline and any extra whitespace.
         return ln.strip()
 
@@ -1610,7 +1622,8 @@
     substitutions = list(extra_substitutions)
     substitutions += getDefaultSubstitutions(test, tmpDir, tmpBase,
                                              normalize_slashes=useExternalSh)
-    script = applySubstitutions(script, substitutions,
+    conditions = { feature: True for feature in test.config.available_features }
+    script = applySubstitutions(script, substitutions, conditions,
                                 recursion_limit=test.config.recursiveExpansionLimit)
 
     return _runShTest(test, litConfig, useExternalSh, script, tmpBase)
diff --git a/llvm/utils/lit/tests/Inputs/shtest-if-else/lit.cfg b/llvm/utils/lit/tests/Inputs/shtest-if-else/lit.cfg
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-if-else/lit.cfg
@@ -0,0 +1,7 @@
+import lit.formats
+config.name = 'shtest-if-else'
+config.test_format = lit.formats.ShTest()
+config.test_source_root = None
+config.test_exec_root = None
+config.suffixes = ['.txt']
+config.available_features.add("feature")
diff --git a/llvm/utils/lit/tests/Inputs/shtest-if-else/test.txt b/llvm/utils/lit/tests/Inputs/shtest-if-else/test.txt
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-if-else/test.txt
@@ -0,0 +1,21 @@
+# RUN: %if feature { echo "test-1" }
+
+# RUN: %if nofeature { echo "fail" } %else { echo "test-2" }
+
+# Spaces inside curly braces are not ignored
+# RUN: echo test-%if feature { 3 } %else { fail }-test
+# RUN: echo test-%if feature { 4 4 } %else { fail }-test
+
+# RUN: %if feature  \
+# RUN:   { echo     \
+# RUN:     "test-5" \
+# RUN:   }
+
+# RUN: %if nofeature       \
+# RUN:   { echo "fail" }   \
+# RUN: %else               \
+# RUN:   { echo "test-6" }
+
+# RUN: echo "test%if feature {} %else {}-7"
+
+# RUN: echo %%if feature { echo "test-8" }
diff --git a/llvm/utils/lit/tests/shtest-if-else.py b/llvm/utils/lit/tests/shtest-if-else.py
new file mode 100644
--- /dev/null
+++ b/llvm/utils/lit/tests/shtest-if-else.py
@@ -0,0 +1,19 @@
+# RUN: %{lit} -v --show-all %{inputs}/shtest-if-else > %t.out
+# RUN: FileCheck --input-file %t.out %s
+# END.
+
+# CHECK: -- Testing:
+# CHECK-NEXT: PASS: shtest-if-else :: test.txt (1 of 1)
+# CHECK-NEXT: Script:
+# CHECK-NEXT: --
+# CHECK-NEXT: 'RUN: at line 1'; echo "test-1"
+# CHECK-NEXT: 'RUN: at line 3'; echo "test-2"
+# CHECK-NEXT: 'RUN: at line 6'; echo test- 3 -test
+# CHECK-NEXT: 'RUN: at line 7'; echo test- 4 4 -test
+# CHECK-NEXT: 'RUN: at line 9'; echo "test-5"
+# CHECK-NEXT: 'RUN: at line 14'; echo "test-6"
+# CHECK-NEXT: 'RUN: at line 19'; echo "test-7"
+# CHECK-NEXT: 'RUN: at line 21'; echo %if feature { echo "test-8" }
+# CHECK-NEXT: --
+# CHECK-NEXT: Exit Code: 0
+