[UTC] CHECK-EMPTY instead of skipping blank lines (#165718)

Previously, any blank lines in IR were ignored by UTC, leading to more
fragile `CHECK`s being generated.

This change lets UTC, 1) emit `CHECK-EMPTY` to check blank lines, and 2)
generate more `CHECK-NEXT`s, landing the discussion
https://github.com/llvm/llvm-project/pull/165419#issuecomment-3457572422.

Moreover, this change also aligns the behavior of IR check-gen to ASM
check-gen, which has been emitting `CHECK-EMPTY` since
https://github.com/llvm/llvm-project/commit/a8a89c77ea3c16b45763fca6940bbfd3bef7884f.

1 files changed, 20 insertions, 3 deletions

diff --git a/llvm/utils/UpdateTestChecks/common.py b/llvm/utils/UpdateTestChecks/common.py
index 119303c..2dad16a 100644
--- a/llvm/utils/UpdateTestChecks/common.py
+++ b/llvm/utils/UpdateTestChecks/common.py
@@ -29,7 +29,7 @@ Version changelog:
    'none' and 'all'. 'smart' is the default.
 5: Basic block labels are matched by FileCheck expressions
 6: The semantics of TBAA checks has been incorporated in the check lines.
-7: Indent switch-cases correctly.
+7: Indent switch-cases correctly; CHECK-EMPTY instead of skipping blank lines.
 """
 DEFAULT_VERSION = 6
 
@@ -2280,6 +2280,14 @@ def add_checks(
             # For IR output, change all defs to FileCheck variables, so we're immune
             # to variable naming fashions.
             else:
+                if ginfo.get_version() >= 7:
+                    # Record the indices of blank lines in the function body preemptively.
+                    blank_line_indices = {
+                        i for i, line in enumerate(func_body) if line.strip() == ""
+                    }
+                else:
+                    blank_line_indices = set()
+
                 func_body = generalize_check_lines(
                     func_body,
                     ginfo,
@@ -2305,9 +2313,18 @@ def add_checks(
 
                 is_blank_line = False
 
-                for func_line in func_body:
+                for idx, func_line in enumerate(func_body):
                     if func_line.strip() == "":
-                        is_blank_line = True
+                        # We should distinguish if the line is a 'fake' blank line generated by
+                        # generalize_check_lines removing comments.
+                        # Fortunately, generalize_check_lines does not change the index of each line,
+                        # we can record the indices of blank lines preemptively.
+                        if idx in blank_line_indices:
+                            output_lines.append(
+                                "{} {}-EMPTY:".format(comment_marker, checkprefix)
+                            )
+                        else:
+                            is_blank_line = True
                         continue
                     if not check_inst_comments:
                         # Do not waste time checking IR comments unless necessary.