[XRay] Synthesize a reference to the xray_instr_map

Without the synthesized reference to a symbol in the xray_instr_map,
linker section garbage collection will helpfully remove the whole
xray_instr_map section from the final executable (or archive). This will
cause the runtime to not be able to identify the sleds and hot-patch the
calls/jumps into the runtime trampolines.

This change adds a reference from the text section at the end of the
function to keep around the associated xray_instr_map section as well.

We also make sure that we catch this reference in the test.

Reviewers: chandlerc, echristo, majnemer, mehdi_amini

Subscribers: mehdi_amini, llvm-commits, dberris

Differential Revision: https://reviews.llvm.org/D23398

llvm-svn: 279204

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index e5e8fe77b10..03c8b1d3cb6 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -1109,7 +1109,19 @@ void X86AsmPrinter::EmitXRayTable() {
       Section = OutContext.getELFSection("xray_instr_map", ELF::SHT_PROGBITS,
                                          ELF::SHF_ALLOC);
     }
+
+    // Before we switch over, we force a reference to a label inside the
+    // xray_instr_map section. Since EmitXRayTable() is always called just
+    // before the function's end, we assume that this is happening after the
+    // last return instruction.
+    //
+    // We then align the reference to 16 byte boundaries, which we determined
+    // experimentally to be beneficial to avoid causing decoder stalls.
+    MCSymbol *Tmp = OutContext.createTempSymbol("xray_synthetic_", true);
+    OutStreamer->EmitCodeAlignment(16);
+    OutStreamer->EmitSymbolValue(Tmp, 8, false);
     OutStreamer->SwitchSection(Section);
+    OutStreamer->EmitLabel(Tmp);
     for (const auto &Sled : Sleds) {
       OutStreamer->EmitSymbolValue(Sled.Sled, 8);
       OutStreamer->EmitSymbolValue(CurrentFnSym, 8);