[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
diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index e5e8fe7..03c8b1d 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -1109,7 +1109,19 @@
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);