Numbered sequentially temporary variables in LLVM IR

Beginners
1

Hi, everyone. I just begin to learn LLVM IR, and have some problems about temporary variables in LLVM IR. When I compile the C code below with clang -emit-llvm -S main.c -o main.ll -O0

// main.c
int foo(int first, int second) {
    return first + second;
}

int a = 5;

int main() {
    int b = 4;
    return foo(a, b);
}

the generated main.ll looks like below

; ModuleID = 'main.cc'
source_filename = "main.cc"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"

@a = dso_local global i32 5, align 4

; Function Attrs: noinline nounwind optnone uwtable
define dso_local i32 @_Z3fooii(i32 %0, i32 %1) #0 {
  %3 = alloca i32, align 4
  %4 = alloca i32, align 4
  store i32 %0, i32* %3, align 4
  store i32 %1, i32* %4, align 4
  %5 = load i32, i32* %3, align 4
  %6 = load i32, i32* %4, align 4
  %7 = add nsw i32 %5, %6
  ret i32 %7
}

; Function Attrs: noinline norecurse nounwind optnone uwtable
define dso_local i32 @main() #1 {
  %1 = alloca i32, align 4
  %2 = alloca i32, align 4
  store i32 0, i32* %1, align 4
  store i32 4, i32* %2, align 4
  %3 = load i32, i32* @a, align 4
  %4 = load i32, i32* %2, align 4
  %5 = call i32 @_Z3fooii(i32 %3, i32 %4)
  ret i32 %5
}

attributes #0 = { noinline nounwind optnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { noinline norecurse nounwind optnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"clang version 10.0.0-4ubuntu1 "}

I’ve noticed that foo function skips the %2 register and main function skips %0 register. But as what LLVM Language Reference Manual says

Unnamed temporaries are numbered sequentially (using a per-function incrementing counter, starting with 0)

If I adjust the unnamed temporaries in foo and execute main.ll with lli, it returns an error

lli: main.ll:5:3: error: instruction expected to be numbered '%3'
  %2 = alloca i32, align 4
  ^

Does %2 in foo register has an internal usage? Hope someone can give some idea

2

I believe %2 is the (unnamed) entry basic-block in this case, and the IR is equivalent to

; ...snip
define dso_local i32 @_Z3fooii(i32 %0, i32 %1) {
2:
  %3 = alloca i32, align 4
; snip...
3

Ohh, that sounds reasonable, and the LLVM Language Reference Manual also has mentioned

Note that basic blocks and unnamed function parameters are included in this numbering

Skipping some basic block labels does cause some problems. Thanks