[valgrind] [Bug 432801] Valgrind 3.16.1 reports a jump based on uninitialized memory somehow related to clang and signals

[Eyal]

https://bugs.kde.org/show_bug.cgi?id=432801

--- Comment #9 from Eyal <eyals...@gmail.com> ---
Oops, I spoke to soon.  It's a bug in clang.  Here's code that you can try:

#include <signal.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

void foo() {
    // Put the garbage number 123 into eax.
    // It's caller-saved so no problem.
    __asm__ ("lea 123, %eax;");

    // xmm variables are all caller-saved, too.
    // Fill them with garbage.
    __asm__ ("movd %eax, %xmm0");
    __asm__ ("punpcklbw %xmm0, %xmm0");
    __asm__ ("punpcklbw %xmm0, %xmm0");
    __asm__ ("punpcklbw %xmm0, %xmm0");
    __asm__ ("punpcklbw %xmm0, %xmm0");

    __asm__ ("movd %eax, %xmm1");
    __asm__ ("punpcklbw %xmm1, %xmm1");
    __asm__ ("punpcklbw %xmm1, %xmm1");
    __asm__ ("punpcklbw %xmm1, %xmm1");
    __asm__ ("punpcklbw %xmm1, %xmm1");

    __asm__ ("movd %eax, %xmm2");
    __asm__ ("punpcklbw %xmm2, %xmm2");
    __asm__ ("punpcklbw %xmm2, %xmm2");
    __asm__ ("punpcklbw %xmm2, %xmm2");
    __asm__ ("punpcklbw %xmm2, %xmm2");

    __asm__ ("movd %eax, %xmm3");
    __asm__ ("punpcklbw %xmm3, %xmm3");
    __asm__ ("punpcklbw %xmm3, %xmm3");
    __asm__ ("punpcklbw %xmm3, %xmm3");
    __asm__ ("punpcklbw %xmm3, %xmm3");

    __asm__ ("movd %eax, %xmm4");
    __asm__ ("punpcklbw %xmm4, %xmm4");
    __asm__ ("punpcklbw %xmm4, %xmm4");
    __asm__ ("punpcklbw %xmm4, %xmm4");
    __asm__ ("punpcklbw %xmm4, %xmm4");
}

int main() {
  char pattern[] = "0123456789";
  pattern[9] = 0;
  const unsigned long plen = strlen(pattern);
  foo();
  size_t hp=0;
  for (size_t i = 0; i < plen; ++i)
    hp += pattern[i];
  volatile int j = 0;
  if (hp==j) {
    j++;
  }
  printf("%ld\n", hp);
  return 1;
}

Run this with and without foo commented and see that the results are different.
 No valgrind need.

Testing on godbolt, I see that the bug is present even through clang 11.0.1

https://godbolt.org/z/6s1rTd

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