[BOLT] Drop -znow requirement for PLT optimization on x86-64 (#178758)

On x86-64, PLT optimization does not require the binary to be linked
with -znow because indirect calls through GOT work correctly with lazy
binding. At runtime, the dynamic linker's resolver will populate the GOT
entry on the first call, just like with a regular PLT call.

This change removes the -znow requirement specifically for x86-64 while
keeping it for other architectures. I haven't checked RISV-V, but it's
still necessary on AArch64.

bolt/docs/CommandLineArgumentReference.md

@@ -724,7 +724,7 @@
 
 - `--plt=<value>`
 
-  Optimize PLT calls (requires linking with -znow)
+  Optimize PLT calls (requires linking with -znow on non-x86 architectures)
   - `none`: do not optimize PLT calls
   - `hot`: optimize executed (hot) PLT calls
   - `all`: optimize all PLT calls

bolt/lib/Passes/PLTCall.cpp

@@ -23,7 +23,9 @@ namespace opts {
 extern cl::OptionCategory BoltOptCategory;
 
 static cl::opt<bolt::PLTCall::OptType>
-    PLT("plt", cl::desc("optimize PLT calls (requires linking with -znow)"),
+    PLT("plt",
+        cl::desc("optimize PLT calls (requires linking with -znow on "
+                 "non-x86 architectures)"),
         cl::init(bolt::PLTCall::OT_NONE),
         cl::values(clEnumValN(bolt::PLTCall::OT_NONE, "none",
                               "do not optimize PLT calls"),
@@ -76,7 +78,12 @@ Error PLTCall::runOnFunctions(BinaryContext &BC) {
   }
 
   if (NumCallsOptimized) {
-    BC.RequiresZNow = true;
+    // On X86-64, PLT optimization does not require -znow because the indirect
+    // call through GOT works correctly with lazy binding. At runtime, the
+    // resolver will populate the GOT entry on first call just like with a
+    // regular PLT call.
+    if (!BC.isX86())
+      BC.RequiresZNow = true;
     BC.outs() << "BOLT-INFO: " << NumCallsOptimized
               << " PLT calls in the binary were optimized.\n";
   }

bolt/test/runtime/X86/plt-call.c

@@ -0,0 +1,14 @@
+// Check that PLT optimization works on X86-64 without -znow.
+
+// RUN: %clang %cflags %s -o %t.exe -Wl,-q
+// RUN: llvm-bolt %t.exe -o %t.bolt --plt=all
+// RUN: %t.bolt | FileCheck %s
+
+// CHECK: Success
+
+#include <stdio.h>
+
+int main() {
+  puts("Success");
+  return 0;
+}