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From: Paul Floyd <paulf@free.fr>
Subject: Safe transposition of logical and operands
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Hi

I'm looking at a Valgrind issue. Full details here

https://bugs.kde.org/show_bug.cgi?id=472329

This code

void foo(std::optional<int64_t> f) {
   std::cout << (f ? *f : 0) << std::endl;
   std::set<int64_t> test;
   test.emplace(0);
   auto it{test.begin()};
   while (it != test.end()) {
     int64_t b{*it};
// Valgrind error on next line
     if (f && *f != b) {
[snip]

int main() { foo(std::nullopt); }

generates (using g++ 12.2 on FreeBSD 13.2 amd64)

	movq	40(%rbx), %rax
Error on next line:
	cmpq	%r14, %rax	# f, SR.252
	je	.L47
	cmpb	$0, -97(%rbp)	#, %sfp
	jne	.L69

Unless I'm completely mistaken cmpq %r14, %rax is the quadword 
comparison of *f != b and cmpb	$0, -97(%rbp) is the 
std::optional::operator bool() comparison with 0. That means that g++ 
has transposed the order of evaluation.


The first cmpq/je generates a

==49599== Conditional jump or move depends on uninitialised value(s)

Valgrind has machinery to detect transformations like this and to 
convert them back into bit accurate bitwise ands.

However, Valgrind thinks that these transpositions can't be done when 
there are potentially trapping memory loads like *f.

So who is right? Is this a safe transformation?

Regards
Paul