[llvm] 301551a - [PDB] Fix unsigned integer overflow

[Nico Weber via llvm-commits]

Author: Nico Weber
Date: 2021-02-26T11:26:53-05:00
New Revision: 301551ae8e8f54fd66867c9411fcc17b2f02be7d

URL: https://github.com/llvm/llvm-project/commit/301551ae8e8f54fd66867c9411fcc17b2f02be7d
DIFF: https://github.com/llvm/llvm-project/commit/301551ae8e8f54fd66867c9411fcc17b2f02be7d.diff

LOG: [PDB] Fix unsigned integer overflow

When building with -fsanitize=unsigned-integer-overflow, this code
causes a diagnostic like:

    ../../llvm/lib/DebugInfo/PDB/Native/GSIStreamBuilder.cpp:159:15:
    runtime error: unsigned integer overflow:
    90 - 229 cannot be represented in type 'unsigned long'

unsigned integer overflow is well defined and it isn't an issue in
practice, but in obscure scenarios (S1.size() small, S2.size over 2GB
on 32-bit systems) it could even be a bug.

So use the usual idiom for implementing cmp functions instead of the
gernally considered buggy idiom :)
See e.g. https://www.gnu.org/software/libc/manual/html_node/Comparison-Functions.html
or https://stackoverflow.com/questions/10996418/efficient-integer-compare-function/10997428#10997428

Differential Revision: https://reviews.llvm.org/D97557

Added: 
    

Modified: 
    llvm/lib/DebugInfo/PDB/Native/GSIStreamBuilder.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/DebugInfo/PDB/Native/GSIStreamBuilder.cpp b/llvm/lib/DebugInfo/PDB/Native/GSIStreamBuilder.cpp
index 52df26b67916..ae91698b925e 100644
--- a/llvm/lib/DebugInfo/PDB/Native/GSIStreamBuilder.cpp
+++ b/llvm/lib/DebugInfo/PDB/Native/GSIStreamBuilder.cpp
@@ -156,7 +156,7 @@ static int gsiRecordCmp(StringRef S1, StringRef S2) {
   size_t RS = S2.size();
   // Shorter strings always compare less than longer strings.
   if (LS != RS)
-    return LS - RS;
+    return (LS > RS) - (LS < RS);
 
   // If either string contains non ascii characters, memcmp them.
   if (LLVM_UNLIKELY(!isAsciiString(S1) || !isAsciiString(S2)))