[X86] Replace action Promote with Custom for operation ISD::SINT_TO_FP

[vrybalov]

Summary

If attribute "use-soft-float"="true" is set then X86ISelLowering.cpp sets
'Promote' action for ISD::SINT_TO_FP operation on type i32.

But 'Promote' action is not proper in this case since lib function
__floatsidf is available for casting from signed int to float type.
Thus Custom action is more suitable here.

If function attribute "use-soft-float"="true" is set then infinite looping
can happen in DAG combining, function visitSINT_TO_FP() replaces SINT_TO_FP
node with UINT_TO_FP node and function combineUIntToFP() replace vice versa in cycle.
The fix prevents it.

[RKSimon]

test/CodeGen/X86/sitofp.ll

4

This test needs improving - you need to add proper FileCheck testing, preferably use the update_llc_test_checks script to check all codegen and cleanup the actual test - I'm not convinced you need all this code to demonstrate the sitofp?

[vrybalov]

test/CodeGen/X86/sitofp.ll

4

Main purpose of the test is to check llc doesn't hang on compilation. Now it hangs.

[vrybalov]

test/CodeGen/X86/sitofp.ll

4

Without "target triple = "i386-unknown-linux-gnu"" the test passes compilation now.

[efriedma]

lib/Target/X86/X86ISelLowering.cpp

238

Normally, you'd use "Expand" to indicate this is always a libcall.

[craig.topper]

test/CodeGen/X86/sitofp.ll

4

This should be sufficient to hit the bug

target triple = "i386-unknown-linux-gnu"

define double @foo(i16 %foo) #0 {
  %conv = zext i16 %foo to i32
  %conv1 = sitofp i32 %conv to double
  ret double %conv1
}

attributes #0 = { "use-soft-float"="true" }

[craig.topper]

lib/Target/X86/X86ISelLowering.cpp

238

I think really the problem is that UINT_TO_FP is defaulting to Legal in this case. So this DAG combine is being allowed to fire. Making SINT_TO_FP Custom stops this, but I think you really want to set UINT_TO_FP to Promote. Or maybe set them both to Expand as Eli suggested. It doesn't necessarily matter because we should be softening the float before we get to legalize ops.

// If the input is a legal type, and SINT_TO_FP is not legal on this target,
// but UINT_TO_FP is legal on this target, try to convert.
if (!TLI.isOperationLegalOrCustom(ISD::SINT_TO_FP, OpVT) &&
    TLI.isOperationLegalOrCustom(ISD::UINT_TO_FP, OpVT)) {
  // If the sign bit is known to be zero, we can change this to UINT_TO_FP.
  if (DAG.SignBitIsZero(N0))
    return DAG.getNode(ISD::UINT_TO_FP, SDLoc(N), VT, N0);
}

[vrybalov]

Updated test.

[spatel]

test/CodeGen/X86/sitofp.ll

4

The general rule is that if you're going to test something, then you might as well test that the output is *correct* rather than *not wrong*. The script that Simon mentioned makes that easy, especially now that the test is minimized. Please use FileCheck and the script for this test.

[vrybalov]

The patch is updated according to review comments.

[craig.topper]

LGTM

[lebedev.ri]

llvm/trunk/test/CodeGen/X86/sitofp.ll

17 ↗

(On Diff #143722)

It would be really nice for all the tests to be committed first,
so that the following codechange commit actually shows the test change.

[vrybalov]

Simon, thank you for the test patching. I did not check it with recent patches. In the future I will be more careful with it.

llvm/trunk/test/CodeGen/X86/sitofp.ll

17 ↗

(On Diff #143722)

It was impossible to commit the test before the patch. LLC hung on the test and did not produce any output.

lib/Target/X86/X86ISelLowering.cpp

if (X86ScalarSSEf32) {
       // f32 and f64 cases are Legal, f80 case is not
       setOperationAction(ISD::SINT_TO_FP     , MVT::i32  , Custom);
     } else {
       setOperationAction(ISD::SINT_TO_FP     , MVT::i16  , Custom);
       setOperationAction(ISD::SINT_TO_FP     , MVT::i32  , Custom);
     }
   } else {
     setOperationAction(ISD::SINT_TO_FP     , MVT::i16  , Promote);
-    setOperationAction(ISD::SINT_TO_FP     , MVT::i32  , Promote);
+    setOperationAction(ISD::SINT_TO_FP     , MVT::i32  , Custom);
   }
 
   // Promote i1/i8 FP_TO_SINT to larger FP_TO_SINTS's, as X86 doesn't have
   // this operation.
   setOperationAction(ISD::FP_TO_SINT       , MVT::i1   , Promote);
   setOperationAction(ISD::FP_TO_SINT       , MVT::i8   , Promote);
 
   if (!Subtarget.useSoftFloat()) {

test/CodeGen/X86/sitofp.ll

+; RUN: llc < %s
+
+target triple = "i386-unknown-linux-gnu"
+
+define double @foo(i16 %a) #0 {
+entry:
+  %conv = zext i16 %a to i32
+  %conv1 = sitofp i32 %conv to double
+  ret double %conv1
+}
+
+attributes #0 = { "use-soft-float"="true" }