[ConstantFolding] Do not remove side effect from constrained functions

[sepavloff]

Summary

According to the discussion in https://reviews.llvm.org/D110322 the code
that removes side effect from replaced function call is deleted.

[nikic]

LG to unblock further work.

llvm/lib/Analysis/ConstantFolding.cpp

 /// \param St Exception flags raised during constant evaluation.
 static bool mayFoldConstrained(ConstrainedFPIntrinsic *CI,
                                APFloat::opStatus St) {
   Optional<RoundingMode> ORM = CI->getRoundingMode();
   Optional<fp::ExceptionBehavior> EB = CI->getExceptionBehavior();
 
   // If the operation does not change exception status flags, it is safe
   // to fold.
-  if (St == APFloat::opStatus::opOK) {
-    // When FP exceptions are not ignored, intrinsic call will not be
-    // eliminated, because it is considered as having side effect. But we
-    // know that its evaluation does not raise exceptions, so side effect
-    // is absent. To allow removing the call, mark it as not accessing memory.
-    if (EB && *EB != fp::ExceptionBehavior::ebIgnore)
-      CI->addFnAttr(Attribute::ReadNone);
+  if (St == APFloat::opStatus::opOK)
     return true;
-  }
 
   // If evaluation raised FP exception, the result can depend on rounding
   // mode. If the latter is unknown, folding is not possible.
   if (!ORM || *ORM == RoundingMode::Dynamic)
     return false;
 
   // If FP exceptions are ignored, fold the call, even if such exception is
   // raised.

llvm/test/Transforms/InstSimplify/constfold-constrained.ll

entry:
   %result = call double @llvm.experimental.constrained.fadd.f64(double 1.0, double 0x3FF0000000000001, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
   ret double %result
 }
 
 ; If result is exact, folding is allowed even if exceptions may be checked.
 define double @fadd_05() #0 {
 ; CHECK-LABEL: @fadd_05(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[RESULT:%.*]] = call double @llvm.experimental.constrained.fadd.f64(double 1.000000e+00, double 2.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
 ; CHECK-NEXT:    ret double 3.000000e+00
 ;
 entry:
   %result = call double @llvm.experimental.constrained.fadd.f64(double 1.0, double 2.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
   ret double %result
 }
 
 ; Dynamic rounding mode does not prevent from folding if the result is exact.
 define double @fadd_06() #0 {
 ; CHECK-LABEL: @fadd_06(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[RESULT:%.*]] = call double @llvm.experimental.constrained.fadd.f64(double 1.000000e+00, double 2.000000e+00, metadata !"round.dynamic", metadata !"fpexcept.strict") #[[ATTR0]]
 ; CHECK-NEXT:    ret double 3.000000e+00
 ;
 entry:
   %result = call double @llvm.experimental.constrained.fadd.f64(double 1.0, double 2.0, metadata !"round.dynamic", metadata !"fpexcept.strict") #0
   ret double %result
 }
 
 ; Inexact results prevents from folding if rounding mode is unknown.

llvm/test/Transforms/InstSimplify/fdiv-strictfp.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -instsimplify -S | FileCheck %s
 
 define float @fdiv_constant_fold() #0 {
 ; CHECK-LABEL: @fdiv_constant_fold(
 ; CHECK-NEXT:    ret float 1.500000e+00
 ;
   %f = call float @llvm.experimental.constrained.fdiv.f32(float 3.0, float 2.0, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
 
   ret float %f
 }
 
 define float @fdiv_constant_fold_strict() #0 {
 ; CHECK-LABEL: @fdiv_constant_fold_strict(
+; CHECK-NEXT:    [[F:%.*]] = call float @llvm.experimental.constrained.fdiv.f32(float 3.000000e+00, float 2.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0:[0-9]+]]
 ; CHECK-NEXT:    ret float 1.500000e+00
 ;
   %f = call float @llvm.experimental.constrained.fdiv.f32(float 3.0, float 2.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
 
   ret float %f
 }
 
 define float @fdiv_constant_fold_strict2() #0 {
 ; CHECK-LABEL: @fdiv_constant_fold_strict2(
-; CHECK-NEXT:    [[F:%.*]] = call float @llvm.experimental.constrained.fdiv.f32(float 2.000000e+00, float 3.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0:[0-9]+]]
+; CHECK-NEXT:    [[F:%.*]] = call float @llvm.experimental.constrained.fdiv.f32(float 2.000000e+00, float 3.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
 ; CHECK-NEXT:    ret float [[F]]
 ;
   %f = call float @llvm.experimental.constrained.fdiv.f32(float 2.0, float 3.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
 
   ret float %f
 }
 
 define float @frem_constant_fold() #0 {
 ; CHECK-LABEL: @frem_constant_fold(
 ; CHECK-NEXT:    ret float 1.000000e+00
 ;
   %f = call float @llvm.experimental.constrained.frem.f32(float 3.0, float 2.0, metadata !"round.tonearest", metadata !"fpexcept.ignore") #0
   ret float %f
 }
 
 define float @frem_constant_fold_strict() #0 {
 ; CHECK-LABEL: @frem_constant_fold_strict(
+; CHECK-NEXT:    [[F:%.*]] = call float @llvm.experimental.constrained.frem.f32(float 3.000000e+00, float 2.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
 ; CHECK-NEXT:    ret float 1.000000e+00
 ;
   %f = call float @llvm.experimental.constrained.frem.f32(float 3.0, float 2.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
   ret float %f
 }
 
 define double @fmul_fdiv_common_operand(double %x, double %y) #0 {
 ; CHECK-LABEL: @fmul_fdiv_common_operand(

llvm/test/Transforms/InstSimplify/strictfp-fadd.ll

 ; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %add = call float @llvm.experimental.constrained.fadd.f32(float 0x7ff8000000000000, float 0x7ff8000000000000, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
   ret float %add
 }
 
 define float @fold_fadd_qnan_qnan_ebstrict() #0 {
 ; CHECK-LABEL: @fold_fadd_qnan_qnan_ebstrict(
+; CHECK-NEXT:    [[ADD:%.*]] = call float @llvm.experimental.constrained.fadd.f32(float 0x7FF8000000000000, float 0x7FF8000000000000, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
 ; CHECK-NEXT:    ret float 0x7FF8000000000000
 ;
   %add = call float @llvm.experimental.constrained.fadd.f32(float 0x7ff8000000000000, float 0x7ff8000000000000, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
   ret float %add
 }
 
 define float @fold_fadd_snan_qnan_ebmaytrap() #0 {
 ; CHECK-LABEL: @fold_fadd_snan_qnan_ebmaytrap(