[llvm] [Float2Int] Fix miscompile with floats that can be converted to large values (PR #85996)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: AtariDreams (AtariDreams)

<details>
<summary>Changes</summary>

There is an issue with Float2Int whenever it has to deal with floats that evaluate to numbers that are over INT_MAX, but under UINT_MAX.

Simply, because the conversion method always assumes a signed integer, this works until the 32 bit number is actually a positive 32 bit number that would be interpreted as a negative number when evaluated as an integer, being put in an i32.

This was messing up the Float2Int conversions and causing miscompiles.

To fix this, tell the APSInt constructor if the value is positive or negative.

---
Full diff: https://github.com/llvm/llvm-project/pull/85996.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/Float2Int.cpp (+2-1) 
- (modified) llvm/test/Transforms/Float2Int/basic.ll (+26) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/Float2Int.cpp b/llvm/lib/Transforms/Scalar/Float2Int.cpp
index ccca8bcc1a56ac..dc72a65c618053 100644
--- a/llvm/lib/Transforms/Scalar/Float2Int.cpp
+++ b/llvm/lib/Transforms/Scalar/Float2Int.cpp
@@ -409,7 +409,8 @@ Value *Float2IntPass::convert(Instruction *I, Type *ToTy) {
     } else if (Instruction *VI = dyn_cast<Instruction>(V)) {
       NewOperands.push_back(convert(VI, ToTy));
     } else if (ConstantFP *CF = dyn_cast<ConstantFP>(V)) {
-      APSInt Val(ToTy->getPrimitiveSizeInBits(), /*isUnsigned=*/false);
+      APSInt Val(ToTy->getPrimitiveSizeInBits(),
+                 !CF->getValueAPF().isNegative());
       bool Exact;
       CF->getValueAPF().convertToInteger(Val,
                                          APFloat::rmNearestTiesToEven,
diff --git a/llvm/test/Transforms/Float2Int/basic.ll b/llvm/test/Transforms/Float2Int/basic.ll
index 2854a83179b7eb..a35c9cd2f3c3e0 100644
--- a/llvm/test/Transforms/Float2Int/basic.ll
+++ b/llvm/test/Transforms/Float2Int/basic.ll
@@ -349,3 +349,29 @@ bogusBB:                                          ; preds = %bogusBB
   %tobool = fcmp une double %inc, 0.000000e+00
   br label %bogusBB
 }
+
+define i32 @pr71958() {
+; CHECK-LABEL: @pr71958(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[X_I:%.*]] = alloca i32, align 4
+; CHECK-NEXT:    call void @llvm.lifetime.start.p0(i64 4, ptr nonnull [[X_I]])
+; CHECK-NEXT:    store volatile i32 1, ptr [[X_I]], align 4
+; CHECK-NEXT:    [[X_I_0_X_I_0_X_0_X_0_X_0__I:%.*]] = load volatile i32, ptr [[X_I]], align 4
+; CHECK-NEXT:    [[CMP_I:%.*]] = icmp sgt i32 [[X_I_0_X_I_0_X_0_X_0_X_0__I]], 0
+; CHECK-NEXT:    [[TMP0:%.*]] = zext i1 [[CMP_I]] to i32
+; CHECK-NEXT:    [[MUL_I1:%.*]] = mul i32 [[TMP0]], -1
+; CHECK-NEXT:    call void @llvm.lifetime.end.p0(i64 4, ptr nonnull [[X_I]])
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %x.i = alloca i32, align 4
+  call void @llvm.lifetime.start.p0(i64 4, ptr nonnull %x.i)
+  store volatile i32 1, ptr %x.i, align 4
+  %x.i.0.x.i.0.x.0.x.0.x.0..i = load volatile i32, ptr %x.i, align 4
+  %cmp.i = icmp sgt i32 %x.i.0.x.i.0.x.0.x.0.x.0..i, 0
+  %conv.i = uitofp i1 %cmp.i to double
+  %mul.i = fmul double %conv.i, 0x41EFFFFFFFE00000
+  %conv1.i = fptoui double %mul.i to i32
+  call void @llvm.lifetime.end.p0(i64 4, ptr nonnull %x.i)
+  ret i32 0
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/85996