[ValueTracking] Compute known FPClass from signbit idiom (#80740)

This patch improves `computeKnownFPClass` by using context-sensitive
information from `DomConditionCache`.
The motivation of this patch is to optimize the following case found in
[fmt/format.h](https://github.com/fmtlib/fmt/blob/e17bc67547a66cdd378ca6a90c56b865d30d6168/include/fmt/format.h#L3555-L3566):
```
define float @test(float %x, i1 %cond) {
  %i32 = bitcast float %x to i32
  %cmp = icmp slt i32 %i32, 0
  br i1 %cmp, label %if.then1, label %if.else

if.then1:
  %fneg = fneg float %x
  br label %if.end

if.else:
  br i1 %cond, label %if.then2, label %if.end

if.then2:
  br label %if.end

if.end:
  %value = phi float [ %fneg, %if.then1 ], [ %x, %if.then2 ], [ %x, %if.else ]
  %ret = call float @llvm.fabs.f32(float %value)
  ret float %ret
}
```
We can prove the sign bit of %value is always zero. Then the fabs can be
eliminated.

This pattern also exists in cpython/duckdb/oiio/openexr.

Compile-time impact:
https://llvm-compile-time-tracker.com/compare.php?from=f82e0809ba12170e2f648f8a1ac01e78ef06c958&to=041218bf5491996edd828cc15b3aec5a59ddc636&stat=instructions:u


|stage1-O3|stage1-ReleaseThinLTO|stage1-ReleaseLTO-g|stage1-O0-g|stage2-O3|stage2-O0-g|stage2-clang|
|--|--|--|--|--|--|--|
|-0.00%|+0.01%|+0.00%|-0.03%|+0.00%|+0.00%|+0.02%|

Author: dtcxzyw

llvm/lib/Analysis/DomConditionCache.cpp

@@ -66,6 +66,11 @@ static void findAffectedValues(Value *Cond,
         // A > C3 && A < C4.
         if (match(A, m_Add(m_Value(X), m_ConstantInt())))
           AddAffected(X);
+        // Handle icmp slt/sgt (bitcast X to int), 0/-1, which is supported by
+        // computeKnownFPClass().
+        if ((Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_SGT) &&
+            match(A, m_ElementWiseBitCast(m_Value(X))))
+          Affected.push_back(X);
       }
     } else if (match(Cond, m_CombineOr(m_FCmp(Pred, m_Value(A), m_Constant()),
                                        m_Intrinsic<Intrinsic::is_fpclass>(

llvm/lib/Analysis/ValueTracking.cpp

@@ -4272,7 +4272,7 @@ static void computeKnownFPClassFromCond(const Value *V, Value *Cond,
   Value *LHS;
   uint64_t ClassVal = 0;
   const APFloat *CRHS;
-  // TODO: handle sign-bit check idiom
+  const APInt *RHS;
   if (match(Cond, m_FCmp(Pred, m_Value(LHS), m_APFloat(CRHS)))) {
     auto [CmpVal, MaskIfTrue, MaskIfFalse] = fcmpImpliesClass(
         Pred, *CxtI->getParent()->getParent(), LHS, *CRHS, LHS != V);
@@ -4282,6 +4282,15 @@ static void computeKnownFPClassFromCond(const Value *V, Value *Cond,
                              m_Value(LHS), m_ConstantInt(ClassVal)))) {
     FPClassTest Mask = static_cast<FPClassTest>(ClassVal);
     KnownFromContext.knownNot(CondIsTrue ? ~Mask : Mask);
+  } else if (match(Cond, m_ICmp(Pred, m_ElementWiseBitCast(m_Value(LHS)),
+                                m_APInt(RHS)))) {
+    bool TrueIfSigned;
+    if (!isSignBitCheck(Pred, *RHS, TrueIfSigned))
+      return;
+    if (TrueIfSigned == CondIsTrue)
+      KnownFromContext.signBitMustBeOne();
+    else
+      KnownFromContext.signBitMustBeZero();
   }
 }
 

llvm/test/Transforms/InstCombine/fpclass-from-dom-cond.ll

@@ -320,3 +320,118 @@ if.else:
   %ret = call i1 @llvm.is.fpclass.f32(float %x, i32 783)
   ret i1 %ret
 }
+
+define float @test_signbit_check(float %x, i1 %cond) {
+; CHECK-LABEL: define float @test_signbit_check(
+; CHECK-SAME: float [[X:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT:    [[I32:%.*]] = bitcast float [[X]] to i32
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I32]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN1:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then1:
+; CHECK-NEXT:    [[FNEG:%.*]] = fneg float [[X]]
+; CHECK-NEXT:    br label [[IF_END:%.*]]
+; CHECK:       if.else:
+; CHECK-NEXT:    br i1 [[COND]], label [[IF_THEN2:%.*]], label [[IF_END]]
+; CHECK:       if.then2:
+; CHECK-NEXT:    br label [[IF_END]]
+; CHECK:       if.end:
+; CHECK-NEXT:    [[VALUE:%.*]] = phi float [ [[FNEG]], [[IF_THEN1]] ], [ [[X]], [[IF_THEN2]] ], [ [[X]], [[IF_ELSE]] ]
+; CHECK-NEXT:    ret float [[VALUE]]
+;
+  %i32 = bitcast float %x to i32
+  %cmp = icmp slt i32 %i32, 0
+  br i1 %cmp, label %if.then1, label %if.else
+
+if.then1:
+  %fneg = fneg float %x
+  br label %if.end
+
+if.else:
+  br i1 %cond, label %if.then2, label %if.end
+
+if.then2:
+  br label %if.end
+
+if.end:
+  %value = phi float [ %fneg, %if.then1 ], [ %x, %if.then2 ], [ %x, %if.else ]
+  %ret = call float @llvm.fabs.f32(float %value)
+  ret float %ret
+}
+
+define float @test_signbit_check_fail(float %x, i1 %cond) {
+; CHECK-LABEL: define float @test_signbit_check_fail(
+; CHECK-SAME: float [[X:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT:    [[I32:%.*]] = bitcast float [[X]] to i32
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I32]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN1:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then1:
+; CHECK-NEXT:    [[FNEG:%.*]] = fneg float [[X]]
+; CHECK-NEXT:    br label [[IF_END:%.*]]
+; CHECK:       if.else:
+; CHECK-NEXT:    br i1 [[COND]], label [[IF_THEN2:%.*]], label [[IF_END]]
+; CHECK:       if.then2:
+; CHECK-NEXT:    [[FNEG2:%.*]] = fneg float [[X]]
+; CHECK-NEXT:    br label [[IF_END]]
+; CHECK:       if.end:
+; CHECK-NEXT:    [[VALUE:%.*]] = phi float [ [[FNEG]], [[IF_THEN1]] ], [ [[FNEG2]], [[IF_THEN2]] ], [ [[X]], [[IF_ELSE]] ]
+; CHECK-NEXT:    [[RET:%.*]] = call float @llvm.fabs.f32(float [[VALUE]])
+; CHECK-NEXT:    ret float [[RET]]
+;
+  %i32 = bitcast float %x to i32
+  %cmp = icmp slt i32 %i32, 0
+  br i1 %cmp, label %if.then1, label %if.else
+
+if.then1:
+  %fneg = fneg float %x
+  br label %if.end
+
+if.else:
+  br i1 %cond, label %if.then2, label %if.end
+
+if.then2:
+  %fneg2 = fneg float %x
+  br label %if.end
+
+if.end:
+  %value = phi float [ %fneg, %if.then1 ], [ %fneg2, %if.then2 ], [ %x, %if.else ]
+  %ret = call float @llvm.fabs.f32(float %value)
+  ret float %ret
+}
+
+define <2 x float> @test_signbit_check_wrong_type(<2 x float> %x, i1 %cond) {
+; CHECK-LABEL: define <2 x float> @test_signbit_check_wrong_type(
+; CHECK-SAME: <2 x float> [[X:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT:    [[I32:%.*]] = bitcast <2 x float> [[X]] to i64
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i64 [[I32]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN1:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then1:
+; CHECK-NEXT:    [[FNEG:%.*]] = fneg <2 x float> [[X]]
+; CHECK-NEXT:    br label [[IF_END:%.*]]
+; CHECK:       if.else:
+; CHECK-NEXT:    br i1 [[COND]], label [[IF_THEN2:%.*]], label [[IF_END]]
+; CHECK:       if.then2:
+; CHECK-NEXT:    br label [[IF_END]]
+; CHECK:       if.end:
+; CHECK-NEXT:    [[VALUE:%.*]] = phi <2 x float> [ [[FNEG]], [[IF_THEN1]] ], [ [[X]], [[IF_THEN2]] ], [ [[X]], [[IF_ELSE]] ]
+; CHECK-NEXT:    [[RET:%.*]] = call <2 x float> @llvm.fabs.v2f32(<2 x float> [[VALUE]])
+; CHECK-NEXT:    ret <2 x float> [[RET]]
+;
+  %i32 = bitcast <2 x float> %x to i64
+  %cmp = icmp slt i64 %i32, 0
+  br i1 %cmp, label %if.then1, label %if.else
+
+if.then1:
+  %fneg = fneg <2 x float> %x
+  br label %if.end
+
+if.else:
+  br i1 %cond, label %if.then2, label %if.end
+
+if.then2:
+  br label %if.end
+
+if.end:
+  %value = phi <2 x float> [ %fneg, %if.then1 ], [ %x, %if.then2 ], [ %x, %if.else ]
+  %ret = call <2 x float> @llvm.fabs.v2f32(<2 x float> %value)
+  ret <2 x float> %ret
+}