wasm-ld: Disabling on-by-default features doesn't work when LTO is used

[alexcrichton]

Given this input:

target triple = "wasm32-unknown-unknown"
target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-n32:64-S128-ni:1:10:20"

define void @foo(ptr %a) #0 {
  call void %a()
  ret void
}

attributes #0 = { "target-cpu"="mvp" }

Locally I see:

$ llvm-as wat.ll
$ wasm-ld wat.bc -o foo.wasm --no-entry --export foo 
$ wasm-tools validate -f=-reference-types foo.wasm
error: func 0 failed to validate

Caused by:
    0: zero byte expected (at offset 0x4b)

This issue is a reduction of rust-lang/rust#130604 and is where a user is trying to disable reference types for their entire compilation and use LTO as well, but it looks like the "target-cpu" isn't taking effect with wasm-ld.

cc @sbc100 do you know what might be causing this?

[sbc100]

That does seem wrong. Just to confirm, if you compile to and object file (not a .bc file) this doesn't fail?

[alexcrichton]

Right yeah, this works ok:

$ llc wat.ll -filetype=obj -o foo.o -mcpu=mvp
$ wasm-ld foo.o -o foo.wasm --no-entry --export foo
$ wasm-tools validate -f=-reference-types foo.wasm

Interestingly the -mcpu=mvp is required though. If I remove that flag then it additionally doesn't work. I forget all the ways that the CPU and such can be configured in LLVM, but I believe that passing -mcpu=mvp mirrors what the Rust compiler does at least. (I'm mostly not sure how the CLI flag -mcpu=mvp interacts with the "target-cpu"="mvp" attribute on functions)

[llvmbot]

@llvm/issue-subscribers-lld-wasm

Author: Alex Crichton (alexcrichton)

Given this input:

target triple = "wasm32-unknown-unknown"
target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-n32:64-S128-ni:1:10:20"

define void @<!-- -->foo(ptr %a) #<!-- -->0 {
  call void %a()
  ret void
}

attributes #<!-- -->0 = { "target-cpu"="mvp" }

Locally I see:

$ llvm-as wat.ll
$ wasm-ld wat.bc -o foo.wasm --no-entry --export foo 
$ wasm-tools validate -f=-reference-types foo.wasm
error: func 0 failed to validate

Caused by:
    0: zero byte expected (at offset 0x4b)

This issue is a reduction of rust-lang/rust#130604 and is where a user is trying to disable reference types for their entire compilation and use LTO as well, but it looks like the "target-cpu" isn't taking effect with wasm-ld.

cc @sbc100 do you know what might be causing this?

[StackOverflowExcept1on]

@sbc100 Any updates on this bug?

[sbc100]

I cannot reproduce this anymore:

$ wasm-tools validate -f=-reference-types foo.wasm
$ wasm-tools validate -f=-reference-types foo_lto.wasm

Also, the two binaries produced via LTO and non-LTO are identical for me:

$ wasm-ld --strip-all foo.o -o foo.wasm --no-entry --export foo
$ wasm-ld --strip-all wat.bc -o foo_lto.wasm --no-entry --export foo
$ diff foo.wasm foo_lto.wasm

Can either of you still reproduce this?

[StackOverflowExcept1on]

@sbc100 With LLVM 20? As far as I know Rust has recently updated to LLVM 20.

[sbc100]

I only tested on tip-of-tree. I don't have LLVM 20 handy to test I'm afraid.

[StackOverflowExcept1on]

I can confirm that this problem is solved at least in nightly Rust

[alexcrichton]

Nice! Seems like this may have been fixed in the 19->20 interim, so closing.

[StackOverflowExcept1on]

@alexcrichton @sbc100 I tried building more complex wasm and now it has memory.fill (bulk memory operations), although my target is wasm32v1-none:

  error: File path: "/gear/target/wasm32-gear/release/demo_ping.wasm"
  |      Unsupported instruction: MemoryFill { mem: 0 }

[alexcrichton]

There's a lot of possible reasons for where that might be coming from, so perhaps it's best to continue the discussion elsewhere given that this specific issue seems to have been fixed? Maybe on your project's issue tracker or similar?

[StackOverflowExcept1on]

@sbc100 Can we reopen this? I'm seeing similar problem again, but so far I have not been able to minimize it. I wrote the steps to reproduce here: rust-lang/rust#140174 (comment).

wasm_program.ll

wget https://gist.githubusercontent.com/StackOverflowExcept1on/a522b4f410da3ed5cd4217aa10715802/raw/0b7aacd8ea92dad23a37cc2035447f55b8c73d28/wasm_program.ll

llvm-as wasm_program.ll
wasm-ld wasm_program.bc -o wasm_program.wasm --no-entry --import-undefined --export init
wasm-tools print wasm_program.wasm

(module $wasm_program.wasm
  (type (;0;) (func (param i32)))
  (type (;1;) (func))
  (import "env" "gr_size" (func $gr_size (;0;) (type 0)))
  (memory (;0;) 2)
  (global $__stack_pointer (;0;) (mut i32) i32.const 66576)
  (export "memory" (memory 0))
  (export "init" (func $init))
  (func $init (;1;) (type 1)
    (local i32 i32)
    global.get $__stack_pointer
    i32.const 16
    i32.sub
    local.tee 0
    global.set $__stack_pointer
    local.get 0
    i32.const 0
    i32.store offset=12
    local.get 0
    i32.const 12
    i32.add
    call $gr_size
    block ;; label = @1
      block ;; label = @2
        local.get 0
        i32.load offset=12
        local.tee 1
        i32.const -1
        i32.le_s
        br_if 0 (;@2;)
        local.get 1
        br_if 1 (;@1;)
        local.get 0
        i32.const 16
        i32.add
        global.set $__stack_pointer
        return
      end
      call $alloc::raw_vec::capacity_overflow::hbcdda363c397d873
      unreachable
    end
    i32.const 0
    i32.load8_u offset=1024
    drop
    call $alloc::alloc::handle_alloc_error::h375d584c5a428f70
    unreachable
  )
  (func $alloc::raw_vec::capacity_overflow::hbcdda363c397d873 (;2;) (type 1)
    call $core::panicking::panic_nounwind_fmt::h965b19ef84cb4c59
    unreachable
  )
  (func $alloc::alloc::handle_alloc_error::h375d584c5a428f70 (;3;) (type 1)
    call $__rustc::__rust_alloc_error_handler
    unreachable
  )
  (func $core::panicking::panic_nounwind_fmt::h965b19ef84cb4c59 (;4;) (type 1)
    loop ;; label = @1
      br 0 (;@1;)
    end
  )
  (func $__rustc::__rust_alloc_error_handler (;5;) (type 1)
    call $__rustc::__rdl_oom
    unreachable
  )
  (func $__rustc::__rdl_oom (;6;) (type 1)
    call $core::panicking::panic_nounwind_fmt::h965b19ef84cb4c59
    unreachable
  )
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  (@producers
    (language "Rust" "")
    (processed-by "rustc" "1.88.0-nightly (d6c1e454a 2025-04-21)")
  )
  (@custom "target_features" (after code) "\08+\0bbulk-memory+\0fbulk-memory-opt+\16call-indirect-overlong+\0amultivalue+\0fmutable-globals+\13nontrapping-fptoint+\0freference-types+\08sign-ext")
)

target_features contains many features that should not appear in wasm32v1-none (mvp + mutable-globals)