[RISCV] Avoid overflow when determining number of nops for code align

RISCVAsmBackend::shouldInsertExtraNopBytesForCodeAlign() assumed that the
align specified would be greater than or equal to the minimum nop length, but
that is not always the case - for example if a user specifies ".align 0" in
assembly.

Differential Revision: https://reviews.llvm.org/D63274
Patch by Edward Jones.

llvm-svn: 366176

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
index 821ac2033c9..ee5f760ebcb 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
@@ -313,8 +313,12 @@ bool RISCVAsmBackend::shouldInsertExtraNopBytesForCodeAlign(
   bool HasStdExtC = STI.getFeatureBits()[RISCV::FeatureStdExtC];
   unsigned MinNopLen = HasStdExtC ? 2 : 4;
 
-  Size = AF.getAlignment() - MinNopLen;
-  return true;
+  if (AF.getAlignment() <= MinNopLen) {
+    return false;
+  } else {
+    Size = AF.getAlignment() - MinNopLen;
+    return true;
+  }
 }
 
 // We need to insert R_RISCV_ALIGN relocation type to indicate the