[llvm] [BOLT] Add --pad-funcs-before=func:n (PR #117924)

[Peter Waller via llvm-commits]

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@@ -319,6 +349,36 @@ bool BinaryEmitter::emitFunction(BinaryFunction &Function,
     Streamer.emitCodeAlignment(Function.getAlign(), &*BC.STI);
   }
 
+  if (size_t Padding = opts::padFunctionBefore(Function)) {
+    // Handle padFuncsBefore after the above alignment logic but before
+    // symbol addresses are decided; with the intent that the nops are
+    // not executed and the original alignment logic is preserved.
+    if (!BC.HasRelocations) {
+      errs() << "BOLT-ERROR: -pad-before-funcs is not supported in "
+             << "non-relocation mode\n";
+      exit(1);
+    }
+
+    // Preserve Function.getMinAlign().
+    if (!isAligned(Function.getMinAlign(), Padding)) {
+      errs() << "BOLT-ERROR: User-requested " << Padding
+             << " padding bytes before function " << Function
+             << " is not a multiple of the minimum function alignment ("
+             << Function.getMinAlign().value() << ").\n";
+      exit(1);
+    }
+
+    LLVM_DEBUG(dbgs() << "BOLT-DEBUG: padding function " << Function << " with "
+                      << Padding << " bytes\n");
+
+    // Since the padding is not executed, it can be null bytes.
+    Streamer.emitFill(Padding, 0);
+
+    Function.setMaxSize(Function.getMaxSize() + Padding);
+    Function.setSize(Function.getSize() + Padding);
+    Function.setImageSize(Function.getImageSize() + Padding);
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peterwaller-arm wrote:

Dropped. This is also residue from an earlier attempt at implementation.

https://github.com/llvm/llvm-project/pull/117924