cd46354dbd
This change is a step towards fixing one long-standing problem with LLVM's debug WASM codegen: excessive use of locals. One local for each temporary value in IR (roughly speaking). This has a lot of problems: 1) It makes it easy to hit engine limitations of 50K locals with certain code patterns and large functions. 2) It makes for larger binaries that are slower to load and slower to compile to native code. 3) It makes certain compilation strategies (spill all WASM locals to stack, for example) for debug code excessively expensive and makes debug WASM code either run very slow, or be less debuggable. 4) It slows down LLVM itself. This change addresses these partially by running a limited version of the stackification pass for unoptimized code, one that gets rid of the most 'obviously' unnecessary locals. Care needs to be taken to not impact LLVM's ability to produce high quality debug variable locations with this pass. To that end: 1) We only allow stackification when it doesn't require moving any instructions. 2) We disable stackification of any locals that are used in DEBUG_VALUEs, or as a frame base. I have verified on a moderately large example that the baseline and the diff produce the same kinds (local/global/stack) of locations, and the only differences are due to the shifting of instruction offsets, with many local.[get|set]s not being present anymore. Even with this quite conservative approach, the results are pretty good: 1) 30% reduction in raw code size, up to 10x reduction in the number of locals for select large methods (~1000 => ~100). 2) ~10% reduction in instructions retired for an "llc -O0" run on a moderately sized input.
90 lines
4.4 KiB
LLVM
90 lines
4.4 KiB
LLVM
; RUN: llc < %s -O0 --filetype=obj -o - | llvm-dwarfdump - | FileCheck %s --check-prefixes DBG_USE
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target triple = "wasm32-unknown-unknown"
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declare i32 @extern_func(i32, i32)
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; We want to produce WASM local "DW_OP_WASM_location 0x00 <local number>" locations
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; in debug code instead of operand stack ("DW_OP_WASM_location 0x02 <depth>") locations
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; since local locations are more widely supported and can cover the entirety of the method.
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; DBG_USE: DW_TAG_subprogram
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; DBG_USE: DW_AT_name ("single_non_dbg_use")
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; DBG_USE: DW_TAG_variable
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; DBG_USE: DW_AT_location
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; DBG_USE: DW_OP_WASM_location 0x0
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; DBG_USE: DW_AT_name ("call_value")
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; DBG_USE: DW_TAG_variable
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; DBG_USE: DW_AT_location
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; DBG_USE: DW_OP_WASM_location 0x0
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; DBG_USE: DW_AT_name ("sub_value")
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define i32 @single_non_dbg_use(i32 %0, i32 %1) !dbg !6 {
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%call_value = call i32 @extern_func(i32 1, i32 2), !dbg !20
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call void @llvm.dbg.value(metadata i32 %call_value, metadata !11, metadata !DIExpression()), !dbg !20
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%div = udiv i32 %0, %1, !dbg !21
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%sub = sub i32 %call_value, %div, !dbg !22
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call void @llvm.dbg.value(metadata i32 %sub, metadata !12, metadata !DIExpression()), !dbg !22
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ret i32 %sub, !dbg !23
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}
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!6 = distinct !DISubprogram(name: "single_non_dbg_use", scope: !1, file: !1, type: !7, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0)
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!7 = !DISubroutineType(types: !8)
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!8 = !{!9, !9, !9}
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!9 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
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!11 = !DILocalVariable(name: "call_value", scope: !6, type: !9)
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!12 = !DILocalVariable(name: "sub_value", scope: !6, type: !9)
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!20 = !DILocation(line: 20, scope: !6)
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!21 = !DILocation(line: 21, scope: !6)
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!22 = !DILocation(line: 22, scope: !6)
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!23 = !DILocation(line: 23, scope: !6)
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; Similarly for a singly-used frame base.
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; DBG_USE: DW_TAG_subprogram
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; DBG_USE: DW_AT_frame_base (DW_OP_WASM_location 0x0
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; DBG_USE: DW_AT_name ("single_use_frame_base")
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; DBG_USE: DW_TAG_variable
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; DBG_USE: DW_AT_location (DW_OP_fbreg +12)
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; DBG_USE: DW_AT_name ("arg_value")
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define i32 @single_use_frame_base(i32 %0, i32 %1) !dbg !13 {
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%arg_loc = alloca i32, !dbg !24
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store i32 %1, ptr %arg_loc, !dbg !25
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call void @llvm.dbg.declare(metadata ptr %arg_loc, metadata !14, metadata !DIExpression()), !dbg !25
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ret i32 %0, !dbg !26
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}
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!13 = distinct !DISubprogram(name: "single_use_frame_base", scope: !1, file: !1, type: !7, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0)
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!14 = !DILocalVariable(name: "arg_value", scope: !13, type: !9)
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!24 = !DILocation(line: 24, scope: !13)
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!25 = !DILocation(line: 25, scope: !13)
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!26 = !DILocation(line: 26, scope: !13)
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; Similarly for multivalue defs... But we can't really test
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; it due to https://github.com/llvm/llvm-project/issues/136506.
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; declare {i32, i32} @extern_func_multivalue(i32, i32)
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;
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; define i32 @single_non_dbg_use_multivalue(i32 %0, i32 %1) !dbg !15 {
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; %full_value = call {i32, i32} @extern_func_multivalue(i32 1, i32 2), !dbg !27
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; %full_value_one = extractvalue {i32, i32} %full_value, 0, !dbg !27
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; %full_value_two = extractvalue {i32, i32} %full_value, 1, !dbg !27
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; %partial_value = call {i32, i32} @extern_func_multivalue(i32 %full_value_one, i32 %full_value_two), !dbg !28
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; call void @llvm.dbg.value(metadata i32 %full_value_two, metadata !16, metadata !DIExpression()), !dbg !28
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; %partial_value_one = extractvalue {i32, i32} %partial_value, 0, !dbg !28
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; %partial_value_two = extractvalue {i32, i32} %partial_value, 1, !dbg !28
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; call void @llvm.dbg.value(metadata i32 %partial_value_two, metadata !17, metadata !DIExpression()), !dbg !28
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; ret i32 %partial_value_one, !dbg !29
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; }
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;
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; !15 = distinct !DISubprogram(name: "single_non_dbg_use_multivalue", scope: !1, file: !1, type: !7, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0)
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; !16 = !DILocalVariable(name: "value_used", scope: !15, type: !9)
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; !17 = !DILocalVariable(name: "value_unused", scope: !15, type: !9)
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; !27 = !DILocation(line: 27, scope: !15)
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; !28 = !DILocation(line: 28, scope: !15)
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; !29 = !DILocation(line: 29, scope: !15)
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!llvm.dbg.cu = !{!0}
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!llvm.module.flags = !{!2, !3}
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!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "LLC", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
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!1 = !DIFile(filename: "test.ll", directory: "")
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!2 = !{i32 7, !"Dwarf Version", i32 4}
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!3 = !{i32 2, !"Debug Info Version", i32 3}
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