llvm-project

55fdeccc - [SDAG][X86] Remove hack needed to avoid missing x87 FPU stack pops (#128055)

[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.