[all-commits] [llvm/llvm-project] 55fdec: [SDAG][X86] Remove hack needed to avoid missing x8...

[Benjamin Maxwell via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 55fdeccc4567bcd4e3f8df0d177195880a194a6a
      https://github.com/llvm/llvm-project/commit/55fdeccc4567bcd4e3f8df0d177195880a194a6a
  Author: Benjamin Maxwell <benjamin.maxwell at arm.com>
  Date:   2025-03-03 (Mon, 03 Mar 2025)

  Changed paths:
    M llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
    M llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/lib/Target/X86/X86ISelLowering.h
    M llvm/lib/Target/X86/X86ISelLoweringCall.cpp
    M llvm/test/CodeGen/PowerPC/llvm.modf.ll

  Log Message:
  -----------
  [SDAG][X86] Remove hack needed to avoid missing x87 FPU stack pops (#128055)

If a (two-result) node like `FMODF` or `FFREXP` is expanded to a library
call, where said library has the function prototype like: `float(float,
float*)` -- that is it returns a float from the call and via an output
pointer. The first result of the node maps to the value returned by
value and the second result maps to the value returned via the output
pointer.

If only the second result is used after the expansion, we hit an issue
on x87 targets:

```
// Before expansion: 
t0, t1 = fmodf x
return t1  // t0 is unused
```

Expanded result:
```
ptr = alloca
ch0 = call modf ptr
t0, ch1 = copy_from_reg, ch0 // t0 unused
t1, ch2 = ldr ptr, ch1
return t1
```

So far things are alright, but the DAGCombiner optimizes this to:
```
ptr = alloca
ch0 = call modf ptr
// copy_from_reg optimized out
t1, ch1 = ldr ptr, ch0
return t1
```

On most targets this is fine. The optimized out `copy_from_reg` is
unused and is a NOP. However, x87 uses a floating-point stack, and if
the `copy_from_reg` is optimized out it won't emit a pop needed to
remove the unused result.

The prior solution for this was to attach the chain from the
`copy_from_reg` to the root, which did work, however, the root is not
always available (it's set to null during legalize types). So the
alternate solution in this patch is to replace the `copy_from_reg` with
an `X86ISD::POP_FROM_X87_REG` within the X86 call lowering. This node is
the same as `copy_from_reg` except this node makes it explicit that it
may lower to an x87 FPU stack pop. Optimizations should be more cautious
when handling this node than a normal CopyFromReg to avoid removing a
required FPU stack pop.

```
ptr = alloca
ch0 = call modf ptr
t0, ch1 = pop_from_x87_reg, ch0 // t0 unused
t1, ch2 = ldr ptr, ch1
return t1
```

Using this node ensures a required x87 FPU pop is not removed due to the
DAGCombiner.

This is an alternate solution for #127976.



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