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.