Difference between revisions of "LDC LLVM profiling instrumentation"
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This page functions as a place to collect information and to document how the implementation in LDC is done. | This page functions as a place to collect information and to document how the implementation in LDC is done. | ||
| − | = Profile-Guided Optimization (PGO) status in LDC = | + | == Profile-Guided Optimization (PGO) status in LDC == |
Work is done in this branch: https://github.com/JohanEngelen/ldc/tree/pgo | Work is done in this branch: https://github.com/JohanEngelen/ldc/tree/pgo | ||
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** If statement | ** If statement | ||
| − | = LLVM InstrProf = | + | == LLVM InstrProf == |
[describe what and how LLVM supports profile instrumentation] | [describe what and how LLVM supports profile instrumentation] | ||
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Branch weights and function entry counts are added to IR (in 2nd compile pass) as metadata: http://llvm.org/docs/doxygen/html/MDBuilder_8h_source.html | Branch weights and function entry counts are added to IR (in 2nd compile pass) as metadata: http://llvm.org/docs/doxygen/html/MDBuilder_8h_source.html | ||
| − | == LLVM intrinsics == | + | === LLVM intrinsics === |
These intrinsics were introduced with [http://reviews.llvm.org/rL223672 LLVM commit r223672], and Clang was modified to use them in [http://reviews.llvm.org/rL223683 Clang commit r223683]. | These intrinsics were introduced with [http://reviews.llvm.org/rL223672 LLVM commit r223672], and Clang was modified to use them in [http://reviews.llvm.org/rL223683 Clang commit r223683]. | ||
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"instrprof_increment" is used to increment control flow counters (function entry, if-statement flow, loops, etc.). | "instrprof_increment" is used to increment control flow counters (function entry, if-statement flow, loops, etc.). | ||
| − | == LLVM runtime calls == | + | === LLVM runtime calls === |
The InstrProf pass adds several external symbols to IR, for example: | The InstrProf pass adds several external symbols to IR, for example: | ||
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compiler-rt's code also takes care of outputting the raw profile data file after execution. | compiler-rt's code also takes care of outputting the raw profile data file after execution. | ||
| − | = Example with Clang 3.7 = | + | == Example with Clang 3.7 == |
The sequence used to [http://clang.llvm.org/docs/UsersManual.html#profiling-with-instrumentation build a program with PGO using clang is]: | The sequence used to [http://clang.llvm.org/docs/UsersManual.html#profiling-with-instrumentation build a program with PGO using clang is]: | ||
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Perhaps it'd be nice for LDC if the llvm-profdata tool is not needed. OTOH, when the profiling data is in a standard LLVM format, all of LLVM's tools can be used to interpret the data. | Perhaps it'd be nice for LDC if the llvm-profdata tool is not needed. OTOH, when the profiling data is in a standard LLVM format, all of LLVM's tools can be used to interpret the data. | ||
| − | == pgo.c == | + | === pgo.c === |
int main(int argc, const char *argv[]) { | int main(int argc, const char *argv[]) { | ||
| Line 74: | Line 74: | ||
} | } | ||
| − | == LLVM IR with clang -fprofile-instr-generate == | + | === LLVM IR with clang -fprofile-instr-generate === |
Unimportant IR has been stripped. | Unimportant IR has been stripped. | ||
| Line 121: | Line 121: | ||
} | } | ||
| − | == pgo0.ll, LLVM IR after clang -fprofile-instr-use == | + | === pgo0.ll, LLVM IR after clang -fprofile-instr-use === |
; Function Attrs: inlinehint nounwind ssp uwtable | ; Function Attrs: inlinehint nounwind ssp uwtable | ||
| Line 150: | Line 150: | ||
| − | == pgo1.ll, LLVM IR after clang -fprofile-instr-use == | + | === pgo1.ll, LLVM IR after clang -fprofile-instr-use === |
The file is identical to pgo0.ll except the last line: | The file is identical to pgo0.ll except the last line: | ||
!3 = !{!"branch_weights", i32 2, i32 1} | !3 = !{!"branch_weights", i32 2, i32 1} | ||
Revision as of 20:35, 23 November 2015
This page functions as a place to collect information and to document how the implementation in LDC is done.
Contents
Profile-Guided Optimization (PGO) status in LDC
Work is done in this branch: https://github.com/JohanEngelen/ldc/tree/pgo
Current status:
- Instrumentation data is output with -fprofile-instr-generate, and can be used with -fprofile-instr-use=<file>
- Instrumentation counters (1st compile pass)
- Function entry
- If statement
- Metadata (2nd compile pass)
- Function entry (+ potentially apply function attribute inlinehint or cold)
- If statement
LLVM InstrProf
[describe what and how LLVM supports profile instrumentation]
slides about PGO in LLVM (from 2013): http://llvm.org/devmtg/2013-11/slides/Carruth-PGO.pdf
Branch weights and function entry counts are added to IR (in 2nd compile pass) as metadata: http://llvm.org/docs/doxygen/html/MDBuilder_8h_source.html
LLVM intrinsics
These intrinsics were introduced with LLVM commit r223672, and Clang was modified to use them in Clang commit r223683.
LLVM features two intrinsics for instrumentation:
The "value_profile" intrinsic is very new, and will not be used for now. "instrprof_increment" is used to increment control flow counters (function entry, if-statement flow, loops, etc.).
LLVM runtime calls
The InstrProf pass adds several external symbols to IR, for example:
@__llvm_profile_runtime = external global i32 ; [#uses = 1]
define linkonce_odr hidden i32 @__llvm_profile_runtime_user() #2 {
%1 = load i32, i32* @__llvm_profile_runtime ; [#uses = 1]
ret i32 %1
}
define internal void @__llvm_profile_init() unnamed_addr #2 {
call void @__llvm_profile_override_default_filename(i8* getelementptr inbounds ([16 x i8], [16 x i8]* @0, i32 0, i32 0))
ret void
}
declare void @__llvm_profile_override_default_filename(i8*)
These need to be added to LDCs runtime. They can be ported from LLVM's compiler-rt/lib/profile, or... I chose to copy LLVM's compiler-rt code. The generated IR by LLVM depends on the target triple. On some platforms, more calls are generated by LLVM so that the runtime can figure out where the profile data (function names, hashes, counters) are located in the binary. On other platforms (Apple for example), it relies on linker magic. compiler-rt's code also takes care of outputting the raw profile data file after execution.
Example with Clang 3.7
The sequence used to build a program with PGO using clang is:
clang -fprofile-instr-generate pgo.c -o pgo ./pgo llvm-profdata merge -output=pgo.profdata default.profraw clang -fprofile-instr-use=pgo.profdata -S -emit-llvm pgo.c -o pgo0.ll ./pgo 1 llvm-profdata merge -output=pgo.profdata default.profraw clang -fprofile-instr-use=pgo.profdata -S -emit-llvm pgo.c pgo1.ll
Perhaps it'd be nice for LDC if the llvm-profdata tool is not needed. OTOH, when the profiling data is in a standard LLVM format, all of LLVM's tools can be used to interpret the data.
pgo.c
int main(int argc, const char *argv[]) {
if (argc)
return 0;
else
return 1;
}
LLVM IR with clang -fprofile-instr-generate
Unimportant IR has been stripped.
@__llvm_profile_name_main = private constant [4 x i8] c"main", section "__DATA,__llvm_prf_names", align 1
@__llvm_profile_counters_main = private global [2 x i64] zeroinitializer, section "__DATA,__llvm_prf_cnts", align 8
@__llvm_profile_data_main = private constant { i32, i32, i64, i8*, i64* } { i32 4, i32 2, i64 10, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @__llvm_profile_name_main, i32 0, i32 0), i64* getelementptr inbounds ([2 x i64], [2 x i64]* @__llvm_profile_counters_main, i32 0, i32 0) }, section "__DATA,__llvm_prf_data", align 8
@__llvm_profile_runtime = external global i32
; Function Attrs: nounwind ssp uwtable
define i32 @main(i32 %argc, i8** %argv) #0 {
entry:
%retval = alloca i32, align 4
%argc.addr = alloca i32, align 4
%argv.addr = alloca i8**, align 8
store i32 0, i32* %retval
store i32 %argc, i32* %argc.addr, align 4
store i8** %argv, i8*** %argv.addr, align 8
%pgocount = load i64, i64* getelementptr inbounds ([2 x i64], [2 x i64]* @__llvm_profile_counters_main, i64 0, i64 0)
%0 = add i64 %pgocount, 1
store i64 %0, i64* getelementptr inbounds ([2 x i64], [2 x i64]* @__llvm_profile_counters_main, i64 0, i64 0)
%1 = load i32, i32* %argc.addr, align 4
%cmp = icmp sgt i32 %1, 1
br i1 %cmp, label %if.then, label %if.else
if.then: ; preds = %entry
%pgocount1 = load i64, i64* getelementptr inbounds ([2 x i64], [2 x i64]* @__llvm_profile_counters_main, i64 0, i64 1)
%2 = add i64 %pgocount1, 1
store i64 %2, i64* getelementptr inbounds ([2 x i64], [2 x i64]* @__llvm_profile_counters_main, i64 0, i64 1)
store i32 0, i32* %retval
br label %return
if.else: ; preds = %entry
store i32 1, i32* %retval
br label %return
return: ; preds = %if.else, %if.then
%3 = load i32, i32* %retval
ret i32 %3
}
; Function Attrs: nounwind declare void @llvm.instrprof.increment(i8*, i64, i32, i32) #1
; Function Attrs: noinline
define linkonce_odr hidden i32 @__llvm_profile_runtime_user() #2 {
%1 = load i32, i32* @__llvm_profile_runtime
ret i32 %1
}
pgo0.ll, LLVM IR after clang -fprofile-instr-use
; Function Attrs: inlinehint nounwind ssp uwtable
define i32 @main(i32 %argc, i8** %argv) #0 !prof !2 {
entry:
%retval = alloca i32, align 4
%argc.addr = alloca i32, align 4
%argv.addr = alloca i8**, align 8
store i32 0, i32* %retval
store i32 %argc, i32* %argc.addr, align 4
store i8** %argv, i8*** %argv.addr, align 8
%0 = load i32, i32* %argc.addr, align 4
%cmp = icmp sgt i32 %0, 1
br i1 %cmp, label %if.then, label %if.else, !prof !3
if.then: ; preds = %entry
store i32 0, i32* %retval
br label %return
if.else: ; preds = %entry
store i32 1, i32* %retval
br label %return
return: ; preds = %if.else, %if.then
%1 = load i32, i32* %retval
ret i32 %1
}
!2 = !{!"function_entry_count", i64 1}
!3 = !{!"branch_weights", i32 1, i32 2}
pgo1.ll, LLVM IR after clang -fprofile-instr-use
The file is identical to pgo0.ll except the last line:
!3 = !{!"branch_weights", i32 2, i32 1}
