[InstCombine] Wrong folding of select + fpext

[dtcxzyw]

Reproducer: https://alive2.llvm.org/ce/z/JPVWbm

; bin/opt -passes=instcombine test.ll -S
define double @f(float %0) {
entry:
  %a5_3 = fcmp olt float %0, 0.000000e+00
  %1 = fpext float %0 to double
  %2 = select i1 %a5_3, double 0.000000e+00, double %1
  ret double %2
}

----------------------------------------
define double @f(float %#0) {
entry:
  %a5_3 = fcmp olt float %#0, 0.000000
  %#1 = fpext float %#0 to double
  %#2 = select i1 %a5_3, double 0.000000, double %#1
  ret double %#2
}
=>
define double @f(float %#0) {
entry:
  %.inv = fcmp ole float %#0, 0.000000
  %#1 = select i1 %.inv, float 0.000000, float %#0
  %#2 = fpext float %#1 to double
  ret double %#2
}
Transformation doesn't verify!

ERROR: Value mismatch

Example:
float %#0 = #x80000000 (-0.0)

Source:
i1 %a5_3 = #x0 (0)
double %#1 = #x8000000000000000 (-0.0)
double %#2 = #x8000000000000000 (-0.0)

Target:
i1 %.inv = #x1 (1)
float %#1 = #x00000000 (+0.0)
double %#2 = #x0000000000000000 (+0.0)
Source value: #x8000000000000000 (-0.0)
Target value: #x0000000000000000 (+0.0)

Summary:
  0 correct transformations
  1 incorrect transformations
  0 failed-to-prove transformations
  0 Alive2 errors

Reported by riscv-tv (riscv-tests/test-160373129.ll). cc @regehr
It is a bit interesting that riscv-tv can also catch miscompilation when optimizing lifted LLVM IR :)

[dtcxzyw]

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

Lines 4280 to 4309 in 32eaf5b

	if (SelectPatternResult::isMinOrMax(SPF)) {
	// Canonicalize so that
	// - type casts are outside select patterns.
	// - float clamp is transformed to min/max pattern
	bool IsCastNeeded = LHS->getType() != SelType;
	Value *CmpLHS = cast<CmpInst>(CondVal)->getOperand(0);
	Value *CmpRHS = cast<CmpInst>(CondVal)->getOperand(1);
	if (IsCastNeeded ||
	(LHS->getType()->isFPOrFPVectorTy() &&
	((CmpLHS != LHS && CmpLHS != RHS) ||
	(CmpRHS != LHS && CmpRHS != RHS)))) {
	CmpInst::Predicate MinMaxPred = getMinMaxPred(SPF, SPR.Ordered);
	Value *Cmp;
	if (CmpInst::isIntPredicate(MinMaxPred))
	Cmp = Builder.CreateICmp(MinMaxPred, LHS, RHS);
	else
	Cmp = Builder.CreateFCmpFMF(MinMaxPred, LHS, RHS,
	cast<Instruction>(SI.getCondition()));
	Value *NewSI = Builder.CreateSelect(Cmp, LHS, RHS, SI.getName(), &SI);
	if (!IsCastNeeded)
	return replaceInstUsesWith(SI, NewSI);
	Value *NewCast = Builder.CreateCast(CastOp, NewSI, SelType);
	return replaceInstUsesWith(SI, NewCast);
	}
	}
	}