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iglosiggio opened this issue Jan 27, 2021 · 2 comments
Closed

Data written at runtime is wrongly classified as const (phase1) #7894

iglosiggio opened this issue Jan 27, 2021 · 2 comments
Labels
bug Observed behavior contradicts documented or intended behavior stage1 The process of building from source via WebAssembly and the C backend.
Milestone
0.11.0

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@iglosiggio
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I think this is the problem behind #6043 and #7878

Example code:

// Writes to rodata
export fn tuple(v: u64) void {
    const bug = .{ .{ v } };
}

// Writes to rodata
export fn anon_struct(v: u64) void {
    const bug = .{ .v = .{ .v = v } };
}

const InnerData = struct {
    v: u64
};
// Writes to rodata
export fn known_inner_struct(v: u64) void {
    const bug = .{ .v = InnerData { .v = v } };
}

const OuterData = struct {
    v: InnerData
};
// Works
export fn known_outer_struct(v: u64) void {
    const works = OuterData { .v = .{ .v = v } };
}

// Works
export fn known_both_structs(v: u64) void {
    const works = OuterData { .v = InnerData { .v = v} };
}

pub fn panic(msg: []const u8, error_return_trace: ?*@import("std").builtin.StackTrace) noreturn {
    @breakpoint();
    unreachable;
}

A snippet of the generated code for the first example (output from objdump because it's a bit less noisy than -femit-asm):

0000000000000030 <tuple>:
  30:	55                   	push   rbp
  31:	48 89 e5             	mov    rbp,rsp
  34:	50                   	push   rax
  35:	48 89 7d f8          	mov    QWORD PTR [rbp-0x8],rdi
  39:	48 8b 45 f8          	mov    rax,QWORD PTR [rbp-0x8]
  3d:	48 89 04 25 00 00 00 	mov    QWORD PTR ds:0x0,rax
  44:	00 
			41: R_X86_64_32S	.rodata.cst8
  45:	48 83 c4 08          	add    rsp,0x8
  49:	5d                   	pop    rbp
  4a:	c3                   	ret    
  4b:	0f 1f 44 00 00       	nop    DWORD PTR [rax+rax*1+0x0]

ds:0x0 is the relocation to be filled with the pointer of .rodata.cst8

IR generation log for the same function:

{
  const bug = {{v    }    };
}
fn tuple() { // (IR)
Entry_0:
  #1  | SrcResetResult        | (unknown)   | - | ResetResult(none)
  #2  | SrcResetResult        | (unknown)   | - | ResetResult(none)
  #3  | SrcConst              | bool        | 2 | false
  #4  | SrcAlloca             | (unknown)   | 1 | Alloca(align=(null),name=bug)
  #5  | SrcResetResult        | (unknown)   | - | ResetResult(var(#4))
  #6  | SrcResolveResult      | (unknown)   | 2 | ResolveResult(var(#4))
  #7  | SrcConst              | usize       | 1 | 0
  #8  | SrcElemPtr            | (unknown)   | 2 | &#6[#7] // no safety
  #9  | SrcResetResult        | (unknown)   | - | ResetResult(inst(#8))
  #10 | SrcResolveResult      | (unknown)   | 2 | ResolveResult(inst(#8))
  #11 | SrcConst              | usize       | 1 | 0
  #12 | SrcElemPtr            | (unknown)   | 2 | &#10[#11] // no safety
  #13 | SrcResetResult        | (unknown)   | - | ResetResult(inst(#12))
  #14 | SrcVarPtr             | (unknown)   | 1 | &v
  #15 | SrcLoadPtr            | (unknown)   | 1 | #14.*
  #16 | SrcEndExpr            | (unknown)   | - | EndExpr(result=inst(#12),value=#15)
  #17 | SrcContainerInitList  | (unknown)   | 1 | {#12}result=#10
  #18 | SrcEndExpr            | (unknown)   | - | EndExpr(result=inst(#8),value=#17)
  #19 | SrcContainerInitList  | (unknown)   | 1 | {#8}result=#6
  #20 | SrcEndExpr            | (unknown)   | - | EndExpr(result=var(#4),value=#19)
  #21 | SrcDeclVar            | (unknown)   | - | const bug = #4 // comptime = #3
  #22 | SrcConst              | void        | 0 | {}
  #23 | SrcConst              | void        | 4 | {}
  #24 | SrcEndExpr            | (unknown)   | - | EndExpr(result=none,value=#23)
  #25 | SrcAddImplicitReturnType| (unknown)   | - | @addImplicitReturnType(#23)
  #26 | SrcResetResult        | (unknown)   | - | ResetResult(return)
  #27 | SrcEndExpr            | (unknown)   | - | EndExpr(result=return,value=#23)
  #28 | SrcReturn             | noreturn    | - | return #23
}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:1:30
~ #1  | SrcResetResult        | (unknown)   | - | ResetResult(none)
-> #2  | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:5
~ #2  | SrcResetResult        | (unknown)   | - | ResetResult(none)
-> #3  | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:5
~ #3  | SrcConst              | bool        | 2 | false
-> #4  | GenConst              | bool        | 0 | false
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:5
~ #4  | SrcAlloca             | (unknown)   | 1 | Alloca(align=(null),name=bug)
-> (null~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:5
~ #5  | SrcResetResult        | (unknown)   | - | ResetResult(var(#4))
-> #5  | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:18
~ #6  | SrcResolveResult      | (unknown)   | 2 | ResolveResult(var(#4))
-> #6  | GenAlloca             | *struct:2:18| 0 | Alloca(align=0,name=bug)
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:21
~ #7  | SrcConst              | usize       | 1 | 0
-> #7  | GenConst              | usize       | 0 | 0
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:21
~ #8  | SrcElemPtr            | (unknown)   | 2 | &#6[#7] // no safety
-> #8  | GenCast               | (* field '0' of struct:2:18)| 0 | NoOp cast #6
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:21
~ #9  | SrcResetResult        | (unknown)   | - | ResetResult(inst(#8))
-> #9  | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:21
~ #10 | SrcResolveResult      | (unknown)   | 2 | ResolveResult(inst(#8))
-> #11 | GenStructFieldPtr     | *struct:2:21| 0 | @StructFieldPtr(&#10.0)
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:23
~ #11 | SrcConst              | usize       | 1 | 0
-> #12 | GenConst              | usize       | 0 | 0
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:23
~ #12 | SrcElemPtr            | (unknown)   | 2 | &#10[#11] // no safety
-> #13 | GenCast               | (* field '0' of struct:2:21)| 0 | NoOp cast #11
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:23
~ #13 | SrcResetResult        | (unknown)   | - | ResetResult(inst(#12))
-> #14 | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:23
~ #14 | SrcVarPtr             | (unknown)   | 1 | &v
-> #15 | GenVarPtr             | *const u64  | 0 | &v
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:23
~ #15 | SrcLoadPtr            | (unknown)   | 1 | #14.*
-> #16 | GenLoadPtr            | u64         | 0 | loadptr(#15)result=(null)
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:23
~ #16 | SrcEndExpr            | (unknown)   | - | EndExpr(result=inst(#12),value=#15)
-> #19 | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:21
~ #17 | SrcContainerInitList  | (unknown)   | 1 | {#12}result=#10
-> #20 | GenLoadPtr            | struct:2:21 | 0 | loadptr(#11)result=(null)
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:21
~ #18 | SrcEndExpr            | (unknown)   | - | EndExpr(result=inst(#8),value=#17)
-> #21 | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:18
~ #19 | SrcContainerInitList  | (unknown)   | 1 | {#8}result=#6
-> #22 | GenConst              | struct:2:18 | 0 | (struct struct:2:18 constant)
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:18
~ #20 | SrcEndExpr            | (unknown)   | - | EndExpr(result=var(#4),value=#19)
-> #23 | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:2:5
~ #21 | SrcDeclVar            | (unknown)   | - | const bug = #4 // comptime = #3
-> #24 | GenDeclVar            | void        | - | const bug: struct:2:18 align(8) = #6
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:1:30
~ #23 | SrcConst              | void        | 4 | {}
-> #25 | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:1:30
~ #24 | SrcEndExpr            | (unknown)   | - | EndExpr(result=none,value=#23)
-> #26 | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:1:30
~ #25 | SrcAddImplicitReturnType| (unknown)   | - | @addImplicitReturnType(#23)
-> #27 | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:1:30
~ #26 | SrcResetResult        | (unknown)   | - | ResetResult(return)
-> #28 | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:1:30
~ #27 | SrcEndExpr            | (unknown)   | - | EndExpr(result=return,value=#23)
-> #30 | GenConst              | void        | 0 | {}
~ /home/ilosiggio/experimentos/zig/zig/build/test/report.zig:1:30
~ #28 | SrcReturn             | noreturn    | - | return #23
append new bb Entry_0
-> (noreturn)
fn tuple() { // (analyzed)
Entry_0:
  #1  | GenReturnPtr          | *void       | 0 | @ReturnPtr
  #8  | GenCast               | (* field '0' of struct:2:18)| 0 | NoOp cast #6
  :6  | GenAlloca             | *struct:2:18| 2 | Alloca(align=0,name=bug)
  #11 | GenStructFieldPtr     | *struct:2:21| 2 | @StructFieldPtr(&#10.0)
  :10 | GenConst              | *struct:2:18| 1 | *(struct struct:2:18 constant)
  #13 | GenCast               | (* field '0' of struct:2:21)| 1 | NoOp cast #11
  #15 | GenVarPtr             | *const u64  | 1 | &v
  #16 | GenLoadPtr            | u64         | 1 | loadptr(#15)result=(null)
  #17 | GenStructFieldPtr     | *u64        | 1 | @StructFieldPtr(&#13.0)
  #18 | GenStorePtr           | void        | - | *#17 = #16
  #20 | GenLoadPtr            | struct:2:21 | 0 | loadptr(#11)result=(null)
  #24 | GenDeclVar            | void        | - | const bug: struct:2:18 align(8) = #6
  #31 | GenReturn             | noreturn    | - | return #25
  :25 | GenConst              | void        | 1 | {}
}

After debugging a lot of hours i think the problem is on translating #10 (SrcResolveResult) into #11 GenStructFieldPtr and :10 (GenConst). Maybe this is an aliasing problem (#18 (GenStorePtr) is marked as ConstValSpecialRuntime but neither #13 (GenCast) nor #11 (GenStructFieldPtr) get recognized as such.

Any guidance on how to understand better the problem is welcome. I know phase1 is not the current focus so i hope this issue doesn't add a lot of noise :)
@iglosiggio
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Seems to be a dupe of #3915

@SpexGuy SpexGuy added bug Observed behavior contradicts documented or intended behavior stage1 The process of building from source via WebAssembly and the C backend. labels Mar 17, 2021
@Vexu Vexu added this to the 0.9.0 milestone Mar 19, 2021
@andrewrk andrewrk modified the milestones: 0.9.0, 0.10.0 May 19, 2021
@mikdusan
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mikdusan commented Jan 2, 2023

looks like no more .rodata in function:

$ zig version
0.11.0-dev.1026+4172c2916

$ zig build-obj bug1.zig

$ objdump -M intel -d bug1.o | sed -n '/<tuple>/,/^$/p'
0000000000000000 <tuple>:
   0:	55                   	push   rbp
   1:	48 89 e5             	mov    rbp,rsp
   4:	48 83 ec 18          	sub    rsp,0x18
   8:	48 89 7d e8          	mov    QWORD PTR [rbp-0x18],rdi
   c:	48 89 7d f0          	mov    QWORD PTR [rbp-0x10],rdi
  10:	48 8b 45 f0          	mov    rax,QWORD PTR [rbp-0x10]
  14:	48 89 45 f8          	mov    QWORD PTR [rbp-0x8],rax
  18:	48 83 c4 18          	add    rsp,0x18
  1c:	5d                   	pop    rbp
  1d:	c3                   	ret
  1e:	66 90                	xchg   ax,ax

@mikdusan mikdusan closed this as completed Jan 2, 2023
@andrewrk andrewrk modified the milestones: 0.12.0, 0.11.0 Jan 3, 2023
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bug Observed behavior contradicts documented or intended behavior stage1 The process of building from source via WebAssembly and the C backend.
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Milestone
0.11.0
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@andrewrk @iglosiggio @SpexGuy @Vexu @mikdusan