LTO miscompilation on some rust<->C/C++ calls

[glandium]

The Rust compiler doesn't create IR identical to clang for the same ABI: rust-lang/rust#102174. When doing cross-language LTO, this leads LLVM to do the wrong things when it inlines those calls. This is a regression from 6c8adc5.

STR:

• Create the following files:
  foo.ll (derived from rustc output, see further below for original source)

target datalayout = "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-f64:32:64-f80:32-n8:16:32-S128"
target triple = "i686-unknown-linux-gnu"

%Foo = type { i32 }
define i32 @foo (ptr byval(%Foo) %foo) {
  %1 = load i32, ptr %foo, align 4
  ret i32 %1
}

bar.ll (derived from clang output, see further below for original source)

target datalayout = "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-f64:32:64-f80:32-n8:16:32-S128"
target triple = "i686-unknown-linux-gnu"

define i32 @bar(i32 %0) {
  %2 = tail call i32 @foo(i32 %0)
  ret i32 %2
}

declare i32 @foo(i32)

• llvm-as foo.ll
• llvm-as bar.ll
• llvm-lto foo.bc bar.bc -filetype=asm -exported-symbol=bar -o -

	.text
	.file	"ld-temp.o"
	.globl	bar
	.p2align	4, 0x90
	.type	bar,@function
bar:
	movl	4(%esp), %eax
	movl	(%eax), %eax
	retl
.Lfunc_end0:
	.size	bar, .Lfunc_end0-bar

	.section	".note.GNU-stack","",@progbits

movl (%eax), %eax shouldn't be there, it's dereferencing something that is not originally a pointer.

What happens is that InstCombine creates the mess by handling the i32->byval conversion as a i32->ptr conversion:

*** IR Dump After InstCombinePass on bar ***
; Function Attrs: mustprogress nofree nosync readnone willreturn
define i32 @bar(i32 %0) local_unnamed_addr #1 {
  %2 = inttoptr i32 %0 to ptr
  %3 = tail call i32 @foo(ptr %2)
  ret i32 %3
}

It should be something like

%2 = alloca i32
store i32 %0, i32* %2

instead of inttoptr, or it should bail out (i.e. what happened before 6c8adc5).

Patch for the latter incoming.

Original source:
foo.rs

// rustc --emit=llvm-ir foo.rs --crate-type=lib --target=i686-unknown-linux-gnu
#[repr(C)]
pub struct Foo(pub u32);

#[no_mangle]
pub unsafe extern "C" fn foo(foo: Foo) -> u32 {
    foo.0
}

bar.c

// clang -O3 -o - -S bar.c -emit-llvm --target=i686-unknown-linux-gnu
struct Foo { int a; };

extern int foo(struct Foo f);

int bar(struct Foo f) {
    return foo(f);
}

[glandium]

https://reviews.llvm.org/D135738

[glandium]

@tru can this be added to the 15.0.4 milestone?

[tru]

When the fix has landed in main. Run the cherry pick comment command to queue up a pick to the release branch.

[tru]

/cherry-pick 86e57e6

[llvmbot]

/branch llvm/llvm-project-release-prs/issue58307

[llvmbot]

/pull-request llvm/llvm-project-release-prs#199