[PATCH] D13979: Introduction of FeatureX87

[Andrey Turetskiy via llvm-commits]

aturetsk added inline comments.

================
Comment at: lib/Target/X86/X86ISelLowering.cpp:527
@@ -525,3 +526,3 @@
 
-  if (!Subtarget.useSoftFloat() && X86ScalarSSEf64) {
+  if (UseX87 && X86ScalarSSEf64) {
     // f32 and f64 use SSE.
----------------
The reason why the float store has appeared is that when we lower call (to the soft float function in this case) we use f80 type for float return if we have SSE. Here is the code snippet from X86ISelLowering.cpp:2428:
```
    // If we prefer to use the value in xmm registers, copy it out as f80 and
    // use a truncate to move it from fp stack reg to xmm reg.
    bool RoundAfterCopy = false;
    if ((VA.getLocReg() == X86::FP0 || VA.getLocReg() == X86::FP1) &&
        isScalarFPTypeInSSEReg(VA.getValVT())) {
      CopyVT = MVT::f80;
      RoundAfterCopy = (CopyVT != VA.getLocVT());
    }
```
The comment in the code explains what's happened. Thus, currently even when we use SSE2 we still rely on the fact that we have X87 and changing that is not trivial.

Now looking at this I'm really inclined to follow Simons advice to inherit FeatureX87 in FeatureMMX. This way SSE/SSE2 will imply X87 which makes a perfect sense since they rely on it.

What do you think?

================
Comment at: test/CodeGen/X86/x87.ll:32
@@ +31,3 @@
+; X87: fild
+; NOX87: __floatdidf
+  %tmp4 = sitofp i64 %l to double
----------------
Done.


http://reviews.llvm.org/D13979