• Hi, I do not understand the way clang is using PHINodes in the second example below. • Let’s consider a condition: ((i < 5) || ((j < 5) || ((k < 5) || (l < 5)))) The resulting IR is: ; ModuleID = ‘Example.cpp’ source_filename = “Example.cpp” target datalayout = “e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128” target triple = “x86_64-redhat-linux-gnu” @.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1 ; Function Attrs: mustprogress noinline optnone uwtable define dso_local void @_Z5Printi(i32 noundef %0) #0 { %2 = alloca i32, align 4 store i32 %0, ptr %2, align 4 %3 = load i32, ptr %2, align 4 %4 = call i32 (ptr, …) @printf(ptr noundef @.str, i32 noundef %3) ret void } declare dso_local i32 @printf(ptr noundef, …) #1 ; Function Attrs: mustprogress noinline norecurse optnone uwtable define dso_local noundef i32 @main() #2 { %1 = alloca i32, align 4 %2 = alloca i32, align 4 %3 = alloca i32, align 4 %4 = alloca i32, align 4 %5 = alloca i32, align 4 %6 = alloca i32, align 4 %7 = alloca i32, align 4 %8 = alloca i32, align 4 %9 = alloca i32, align 4 store i32 0, ptr %1, align 4 store i32 -5, ptr %2, align 4 store i32 -5, ptr %3, align 4 store i32 -5, ptr %4, align 4 store i32 -5, ptr %5, align 4 store i32 -5, ptr %6, align 4 store i32 -5, ptr %7, align 4 store i32 -5, ptr %8, align 4 store i32 -5, ptr %9, align 4 br label %10 10: ; preds = %37, %0 %11 = load i32, ptr %2, align 4 %12 = icmp slt i32 %11, 5 br i1 %12, label %26, label

Article Summaries:

  • The question asks why Clang’s LLVM IR for a nested logical‑OR expression uses several PHI nodes. The example condition is ((i < 5) || ((j < 5) || ((k < 5) || (l < 5)))). The posted IR shows a loop that evaluates each comparison, branches on the result, and then merges the boolean outcomes with PHI nodes (%23, %25, %27). Each PHI node represents the short‑circuit logic: if an earlier test is true, later tests are skipped, and the true value propagates. The user seeks a clear explanation of how these PHI nodes implement the OR semantics in the generated IR.

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