[PATCH] D132482: RISCV: permit unaligned nop-slide padding emission

[Saleem Abdulrasool via Phabricator via llvm-commits]

compnerd updated this revision to Diff 454882.
compnerd marked 3 inline comments as done.
compnerd added a comment.

Address feedback from @jrtc27


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D132482/new/

https://reviews.llvm.org/D132482

Files:
  llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
  llvm/test/MC/RISCV/nop-slide.ll


Index: llvm/test/MC/RISCV/nop-slide.ll
===================================================================
--- /dev/null
+++ llvm/test/MC/RISCV/nop-slide.ll
@@ -0,0 +1,13 @@
+; RUN: llc -mtriple riscv64 -filetype obj -o - %s | llvm-objdump -t - | FileCheck %s
+
+target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n64-S128"
+
+; Ensure that we properly handle the emission of nops into sections which may be
+; classified as text but are actual data.  The alignment for `c` requires the
+; emission of 7 bytes of nops.
+
+ at b = dso_local global i8 0, section ".init.data", align 1
+ at c = dso_local global ptr null, section ".init.data", align 8
+
+; CHECK: 0000000000000000 g     O .init.data	0000000000000001 b
+; CHECK: 0000000000000008 g     O .init.data	0000000000000008 c
Index: llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
===================================================================
--- llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
+++ llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
@@ -354,20 +354,21 @@

 bool RISCVAsmBackend::writeNopData(raw_ostream &OS, uint64_t Count,
                                    const MCSubtargetInfo *STI) const {
-  bool HasStdExtC = STI->getFeatureBits()[RISCV::FeatureStdExtC];
-  unsigned MinNopLen = HasStdExtC ? 2 : 4;
-
-  if ((Count % MinNopLen) != 0)
-    return false;
-
   // The canonical nop on RISC-V is addi x0, x0, 0.
   for (; Count >= 4; Count -= 4)
     OS.write("\x13\0\0\0", 4);

   // The canonical nop on RVC is c.nop.
-  if (Count && HasStdExtC)
-    OS.write("\x01\0", 2);
-
+  if (STI->getFeatureBits()[RISCV::FeatureStdExtC])
+    for (; Count >= 2; Count -= 2)
+      OS.write("\x01\0", 2);
+
+  // If the count is not 4-byte aligned (or 2-byte under the C extensions), we
+  // must be writing data into the text section (otherwise we have unaligned
+  // instructions, and thus have far bigger problems), so just write zeros
+  // instead.  This also may occur for non-standard data sections which are not
+  // declared and thus get detected as text rather than data.
+  OS.write_zeros(Count);
   return true;
 }



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