[XRay] Reduce synthetic references emitted by XRay

[dberris]

Summary

When we're building with XRay instrumentation, we use a trick that
preserves references from the function to a function sled index. This
index table lives in a separate section, and without this trick the
linker is free to garbage-collect this section and all the segments it
refers to. Until we're able to tell the linkers to preserve these
sections, we use this reference trick to keep around both the index and
the entries in the instrumentation map.

Before this change we emitted both a synthetic reference to the label in
the instrumentation map, and to the entry in the function map index.
This change removes the first synthetic reference and only emits one
synthetic reference to the index -- the index entry has the references
to the labels in the instrumentation map, so the linker will still
preserve those if the function itself is preserved.

This reduces the amount of synthetic references we emit from 16 bytes to
just 8 bytes in x86_64, and similarly to other platforms.

[dblaikie]

Roughly makes sense to me.

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

2833–2834 ↗

(On Diff #102991)

Do these both need to be link-time (or even runtime, I guess) relocations? Or could the 'end' be stored as "end - start" (ie: length rather than end) to remove an extra relocation from the object file?

[dberris]

Thanks -- landing this now, we can change more of the details later.

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

2833–2834 ↗

(On Diff #102991)

We can change this, but then that means we're going to have to coordinate it between how we're using the index in the runtime and the back-end (here). Is the concern the effect on linking time? Or do these symbols pollute the debug info somehow?

llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

InstMap = OutContext.getMachOSection("__DATA", "xray_instr_map", 0,
                                          SectionKind::getReadOnlyWithRel());
     FnSledIndex = OutContext.getMachOSection("__DATA", "xray_fn_idx", 0,
                                              SectionKind::getReadOnlyWithRel());
   } else {
     llvm_unreachable("Unsupported target");
   }
 
   // Before we switch over, we force a reference to a label inside the
-  // xray_instr_map and xray_fn_idx sections. Since this function is always
-  // called just before the function's end, we assume that this is happening
-  // after the last return instruction. We also use the synthetic label in the
-  // xray_inster_map as a delimeter for the range of sleds for this function in
-  // the index.
+  // xray_fn_idx sections. This makes sure that the xray_fn_idx section is kept
+  // live by the linker if the function is not garbage-collected. Since this
+  // function is always called just before the function's end, we assume that
+  // this is happening after the last return instruction.
   auto WordSizeBytes = MAI->getCodePointerSize();
-  MCSymbol *SledsStart = OutContext.createTempSymbol("xray_synthetic_", true);
   MCSymbol *IdxRef = OutContext.createTempSymbol("xray_fn_idx_synth_", true);
   OutStreamer->EmitCodeAlignment(16);
-  OutStreamer->EmitSymbolValue(SledsStart, WordSizeBytes, false);
   OutStreamer->EmitSymbolValue(IdxRef, WordSizeBytes, false);
 
   // Now we switch to the instrumentation map section. Because this is done
   // per-function, we are able to create an index entry that will represent the
   // range of sleds associated with a function.
+  MCSymbol *SledsStart = OutContext.createTempSymbol("xray_sleds_start", true);
   OutStreamer->SwitchSection(InstMap);
   OutStreamer->EmitLabel(SledsStart);
   for (const auto &Sled : Sleds)
     Sled.emit(WordSizeBytes, OutStreamer.get(), CurrentFnSym);
-  MCSymbol *SledsEnd = OutContext.createTempSymbol("xray_synthetic_end", true);
+  MCSymbol *SledsEnd = OutContext.createTempSymbol("xray_sleds_end", true);
   OutStreamer->EmitLabel(SledsEnd);
 
   // We then emit a single entry in the index per function. We use the symbols
   // that bound the instrumentation map as the range for a specific function.
   // Each entry here will be 2 * word size aligned, as we're writing down two
   // pointers. This should work for both 32-bit and 64-bit platforms.
   OutStreamer->SwitchSection(FnSledIndex);
   OutStreamer->EmitCodeAlignment(2 * WordSizeBytes);

llvm/trunk/test/CodeGen/AArch64/xray-attribute-instrumentation.ll

 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-LABEL: Ltmp1:
 ; CHECK-NEXT:  ret
 }
 ; CHECK:       .p2align 4
-; CHECK-NEXT:  .xword .Lxray_synthetic_0
 ; CHECK-NEXT:  .xword .Lxray_fn_idx_synth_0
 ; CHECK-NEXT:  .section xray_instr_map,{{.*}}
-; CHECK-LABEL: Lxray_synthetic_0:
+; CHECK-LABEL: Lxray_sleds_start0
 ; CHECK:       .xword .Lxray_sled_0
 ; CHECK:       .xword .Lxray_sled_1
+; CHECK-LABEL: Lxray_sleds_end0

llvm/trunk/test/CodeGen/AArch64/xray-tail-call-sled.ll

 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-LABEL: .Ltmp1:
 ; CHECK-NEXT:  ret
 }
 ; CHECK:       .p2align 4
-; CHECK-NEXT:  .xword .Lxray_synthetic_0
 ; CHECK-NEXT:  .xword .Lxray_fn_idx_synth_0
 ; CHECK-NEXT:  .section xray_instr_map,{{.*}}
-; CHECK-LABEL: Lxray_synthetic_0:
+; CHECK-LABEL: Lxray_sleds_start0:
 ; CHECK:       .xword .Lxray_sled_0
 ; CHECK:       .xword .Lxray_sled_1
-; CHECK-LABEL: Lxray_synthetic_end0:
+; CHECK-LABEL: Lxray_sleds_end0:
 ; CHECK:       .section xray_fn_idx,{{.*}}
 ; CHECK-LABEL: Lxray_fn_idx_synth_0:
-; CHECK:       .xword .Lxray_synthetic_0
-; CHECK-NEXT:  .xword .Lxray_synthetic_end0
+; CHECK:       .xword .Lxray_sleds_start0
+; CHECK-NEXT:  .xword .Lxray_sleds_end0
 
 define i32 @caller() nounwind noinline uwtable "function-instrument"="xray-always" {
 ; CHECK:       .p2align 2
-; CHECK-LABEL: .Lxray_sled_2:
+; CHECK-LABEL: Lxray_sled_2:
 ; CHECK-NEXT:  b  #32
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-LABEL: .Ltmp2:
 ; CHECK:       .p2align 2
-; CHECK-LABEL: .Lxray_sled_3:
+; CHECK-LABEL: Lxray_sled_3:
 ; CHECK-NEXT:  b  #32
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-LABEL: .Ltmp3:
   %retval = tail call i32 @callee()
 ; CHECK:       b  callee
   ret i32 %retval
 }
 ; CHECK:       .p2align 4
-; CHECK-NEXT:  .xword .Lxray_synthetic_1
 ; CHECK-NEXT:  .xword .Lxray_fn_idx_synth_1
 ; CHECK-NEXT:  .section xray_instr_map,{{.*}}
-; CHECK-LABEL: Lxray_synthetic_1:
+; CHECK-LABEL: Lxray_sleds_start1:
 ; CHECK:       .xword .Lxray_sled_2
 ; CHECK:       .xword .Lxray_sled_3
-; CHECK-LABEL: Lxray_synthetic_end1:
+; CHECK-LABEL: Lxray_sleds_end1:
 ; CHECK:       .section xray_fn_idx,{{.*}}
 ; CHECK-LABEL: Lxray_fn_idx_synth_1:
-; CHECK:       .xword .Lxray_synthetic_1
-; CHECK-NEXT:  .xword .Lxray_synthetic_end1
+; CHECK:       .xword .Lxray_sleds_start1
+; CHECK-NEXT:  .xword .Lxray_sleds_end1

llvm/trunk/test/CodeGen/ARM/xray-armv6-attribute-instrumentation.ll

 ; CHECK-NEXT:  mov  r0, r0
 ; CHECK-NEXT:  mov  r0, r0
 ; CHECK-NEXT:  mov  r0, r0
 ; CHECK-NEXT:  mov  r0, r0
 ; CHECK-LABEL: Ltmp1:
 ; CHECK-NEXT:  bx lr
 }
 ; CHECK:       .p2align 4
-; CHECK-NEXT:  .long {{.*}}Lxray_synthetic_0
 ; CHECK-NEXT:  .long {{.*}}Lxray_fn_idx_synth_0
 ; CHECK-NEXT:  .section {{.*}}xray_instr_map{{.*}}
-; CHECK-LABEL: Lxray_synthetic_0:
+; CHECK-LABEL: Lxray_sleds_start0:
 ; CHECK:       .long {{.*}}Lxray_sled_0
 ; CHECK:       .long {{.*}}Lxray_sled_1
-; CHECK-LABEL: Lxray_synthetic_end0:
+; CHECK-LABEL: Lxray_sleds_end0:
 ; CHECK:       .section {{.*}}xray_fn_idx{{.*}}
 ; CHECK-LABEL: Lxray_fn_idx_synth_0:
-; CHECK:       .long {{.*}}Lxray_synthetic_0
-; CHECK-NEXT:  .long {{.*}}Lxray_synthetic_end0
+; CHECK:       .long {{.*}}Lxray_sleds_start0
+; CHECK-NEXT:  .long {{.*}}Lxray_sleds_end0

llvm/trunk/test/CodeGen/ARM/xray-armv7-attribute-instrumentation.ll

 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-NEXT:  nop
 ; CHECK-LABEL: Ltmp1:
 ; CHECK-NEXT:  bx lr
 }
 ; CHECK:       .p2align 4
-; CHECK-NEXT:  .long {{.*}}Lxray_synthetic_0
 ; CHECK-NEXT:  .long {{.*}}Lxray_fn_idx_synth_0
 ; CHECK-NEXT:  .section {{.*}}xray_instr_map{{.*}}
-; CHECK-LABEL: Lxray_synthetic_0:
+; CHECK-LABEL: Lxray_sleds_start0:
 ; CHECK:       .long {{.*}}Lxray_sled_0
 ; CHECK:       .long {{.*}}Lxray_sled_1
-; CHECK-LABEL: Lxray_synthetic_end0:
+; CHECK-LABEL: Lxray_sleds_end0:
 ; CHECK:       .section {{.*}}xray_fn_idx{{.*}}
 ; CHECK-LABEL: Lxray_fn_idx_synth_0:
-; CHECK:       .long {{.*}}xray_synthetic_0
-; CHECK-NEXT:  .long {{.*}}xray_synthetic_end0
+; CHECK:       .long {{.*}}xray_sleds_start0
+; CHECK-NEXT:  .long {{.*}}xray_sleds_end0
 

llvm/trunk/test/CodeGen/X86/xray-attribute-instrumentation.ll

 ; RUN: llc -filetype=asm -o - -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
 ; RUN: llc -filetype=asm -o - -mtriple=x86_64-darwin-unknown    < %s | FileCheck %s
 
 define i32 @foo() nounwind noinline uwtable "function-instrument"="xray-always" {
 ; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_0:
 ; CHECK-NEXT:  .ascii "\353\t"
 ; CHECK-NEXT:  nopw 512(%rax,%rax)
 ; CHECK-LABEL: Ltmp0:
   ret i32 0
 ; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_1:
 ; CHECK-NEXT:  retq
 ; CHECK-NEXT:  nopw %cs:512(%rax,%rax)
 }
 ; CHECK:       .p2align 4, 0x90
-; CHECK-NEXT:  .quad {{.*}}xray_synthetic_0
 ; CHECK-NEXT:  .quad {{.*}}xray_fn_idx_synth_0
 ; CHECK-NEXT:  .section {{.*}}xray_instr_map
-; CHECK-LABEL: Lxray_synthetic_0:
+; CHECK-LABEL: Lxray_sleds_start0:
 ; CHECK:       .quad {{.*}}xray_sled_0
 ; CHECK:       .quad {{.*}}xray_sled_1
-; CHECK-LABEL: Lxray_synthetic_end0:
+; CHECK-LABEL: Lxray_sleds_end0:
 ; CHECK:       .section {{.*}}xray_fn_idx
 ; CHECK-LABEL: Lxray_fn_idx_synth_0:
-; CHECK:       .quad {{.*}}xray_synthetic_0
-; CHECK-NEXT:  .quad {{.*}}xray_synthetic_end0
+; CHECK:       .quad {{.*}}xray_sleds_start0
+; CHECK-NEXT:  .quad {{.*}}xray_sleds_end0
 
 
 ; We test multiple returns in a single function to make sure we're getting all
 ; of them with XRay instrumentation.
 define i32 @bar(i32 %i) nounwind noinline uwtable "function-instrument"="xray-always" {
 ; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_2:
 ; CHECK-NEXT:  .ascii "\353\t"

 NotEqual:
   ret i32 1
 ; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_4:
 ; CHECK-NEXT:  retq
 ; CHECK-NEXT:  nopw %cs:512(%rax,%rax)
 }
 ; CHECK:       .p2align 4, 0x90
-; CHECK-NEXT:  .quad {{.*}}xray_synthetic_1
 ; CHECK-NEXT:  .quad {{.*}}xray_fn_idx_synth_1
 ; CHECK-NEXT:  .section {{.*}}xray_instr_map
-; CHECK-LABEL: Lxray_synthetic_1:
+; CHECK-LABEL: Lxray_sleds_start1:
 ; CHECK:       .quad {{.*}}xray_sled_2
 ; CHECK:       .quad {{.*}}xray_sled_3
 ; CHECK:       .quad {{.*}}xray_sled_4
-; CHECK-LABEL: Lxray_synthetic_end1:
+; CHECK-LABEL: Lxray_sleds_end1:
 ; CHECK:       .section {{.*}}xray_fn_idx
 ; CHECK-LABEL: Lxray_fn_idx_synth_1:
-; CHECK:       .quad {{.*}}xray_synthetic_1
-; CHECK-NEXT:  .quad {{.*}}xray_synthetic_end1
+; CHECK:       .quad {{.*}}xray_sleds_start1
+; CHECK-NEXT:  .quad {{.*}}xray_sleds_end1

llvm/trunk/test/CodeGen/X86/xray-custom-log.ll

define i32 @fn() nounwind noinline uwtable "function-instrument"="xray-always" {
     ; CHECK-NEXT:  movq {{.*}}, %rdi
     ; CHECK-NEXT:  movq {{.*}}, %rsi
     ; CHECK-NEXT:  movabsq $__xray_CustomEvent, %rax
     ; CHECK-NEXT:  callq *%rax
     ; CHECK-NEXT:  popq %rax
     ret i32 0
 }
 ; CHECK:       .section {{.*}}xray_instr_map
-; CHECK-LABEL: Lxray_synthetic_0:
+; CHECK-LABEL: Lxray_sleds_start0:
 ; CHECK:       .quad {{.*}}xray_event_sled_0
 
 declare void @llvm.xray.customevent(i8*, i32)

llvm/trunk/test/CodeGen/X86/xray-log-args.ll

 ; When logging arguments is specified, emit the entry sled accordingly.
 
 ; RUN: llc -filetype=asm -o - -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
 ; RUN: llc -filetype=asm -o - -mtriple=x86_64-darwin-unknown < %s | FileCheck %s
 
 define i32 @callee(i32 %arg) nounwind noinline uwtable "function-instrument"="xray-always" "xray-log-args"="1" {
   ret i32 %arg
 }
-; CHECK-LABEL: Lxray_synthetic_0:
+; CHECK-LABEL: Lxray_sleds_start0:
 ; CHECK:  .quad {{\.?}}Lxray_sled_0
 ; CHECK:  .quad {{_?}}callee
 ; CHECK:  .byte 3
 ; CHECK:  .byte 1
 ; CHECK:  .{{(zero|space)}} 14
 ; CHECK:  .quad {{\.?}}Lxray_sled_1
 ; CHECK:  .quad {{_?}}callee
 ; CHECK:  .byte 1
 ; CHECK:  .byte 1
 ; CHECK:  .{{(zero|space)}} 14
 
 define i32 @caller(i32 %arg) nounwind noinline uwtable "function-instrument"="xray-always" "xray-log-args"="1" {
   %retval = tail call i32 @callee(i32 %arg)
   ret i32 %retval
 }
-; CHECK-LABEL: Lxray_synthetic_1:
+; CHECK-LABEL: Lxray_sleds_start1:
 ; CHECK:  .quad {{\.?}}Lxray_sled_2
 ; CHECK:  .quad {{_?}}caller
 ; CHECK:  .byte 3
 ; CHECK:  .byte 1
 ; CHECK:  .{{(zero|space)}} 14
 ; CHECK:  .quad {{\.?}}Lxray_sled_3
 ; CHECK:  .quad {{_?}}caller
 ; CHECK:  .byte 2
 ; CHECK:  .byte 1
 ; CHECK:  .{{(zero|space)}} 14

llvm/trunk/test/CodeGen/X86/xray-tail-call-sled.ll

 ; RUN: llc -filetype=asm -o - -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
 ; RUN: llc -filetype=asm -o - -mtriple=x86_64-darwin-unknown    < %s | FileCheck %s
 
 define i32 @callee() nounwind noinline uwtable "function-instrument"="xray-always" {
 ; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_0:
 ; CHECK-NEXT:  .ascii "\353\t"
 ; CHECK-NEXT:  nopw 512(%rax,%rax)
 ; CHECK-LABEL: Ltmp0:
   ret i32 0
 ; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_1:
 ; CHECK-NEXT:  retq
 ; CHECK-NEXT:  nopw %cs:512(%rax,%rax)
 }
 ; CHECK:       .p2align 4, 0x90
-; CHECK-NEXT:  .quad {{.*}}xray_synthetic_0{{.*}}
 ; CHECK-NEXT:  .quad {{.*}}xray_fn_idx_synth_0{{.*}}
 ; CHECK-NEXT:  .section {{.*}}xray_instr_map
-; CHECK-LABEL: Lxray_synthetic_0:
+; CHECK-LABEL: Lxray_sleds_start0:
 ; CHECK:       .quad {{.*}}xray_sled_0
 ; CHECK:       .quad {{.*}}xray_sled_1
-; CHECK-LABEL: Lxray_synthetic_end0:
+; CHECK-LABEL: Lxray_sleds_end0:
 ; CHECK-NEXT:  .section {{.*}}xray_fn_idx
 ; CHECK-LABEL: Lxray_fn_idx_synth_0:
-; CHECK:       .quad {{.*}}xray_synthetic_0
-; CHECK-NEXT:  .quad {{.*}}xray_synthetic_end0
+; CHECK:       .quad {{.*}}xray_sleds_start0
+; CHECK-NEXT:  .quad {{.*}}xray_sleds_end0
 
 define i32 @caller() nounwind noinline uwtable "function-instrument"="xray-always" {
 ; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_2:
 ; CHECK-NEXT:  .ascii "\353\t"
 ; CHECK-NEXT:  nopw 512(%rax,%rax)
 ; CHECK-LABEL: Ltmp1:
 ; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_3:
 ; CHECK-NEXT:  .ascii "\353\t"
 ; CHECK-NEXT:  nopw 512(%rax,%rax)
 ; CHECK-LABEL: Ltmp2:
   %retval = tail call i32 @callee()
 ; CHECK:       jmp {{.*}}callee {{.*}}# TAILCALL
   ret i32 %retval
 }
 ; CHECK:       .p2align 4, 0x90
-; CHECK-NEXT:  .quad {{.*}}xray_synthetic_1{{.*}}
 ; CHECK-NEXT:  .quad {{.*}}xray_fn_idx_synth_1{{.*}}
-; CHECK-LABEL: Lxray_synthetic_1:
+; CHECK-LABEL: Lxray_sleds_start1:
 ; CHECK:       .quad {{.*}}xray_sled_2
 ; CHECK:       .quad {{.*}}xray_sled_3
-; CHECK-LABEL: Lxray_synthetic_end1:
+; CHECK-LABEL: Lxray_sleds_end1:
 ; CHECK:       .section {{.*}}xray_fn_idx
 ; CHECK-LABEL: Lxray_fn_idx_synth_1:
-; CHECK:       .quad {{.*}}xray_synthetic_1
-; CHECK:       .quad {{.*}}xray_synthetic_end1
+; CHECK:       .quad {{.*}}xray_sleds_start1
+; CHECK:       .quad {{.*}}xray_sleds_end1