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Subject: [Bug target/29042]  New: Useless floating-point stores and loads on x86
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From: "guillaume dot melquiond at ens-lyon dot fr" <gcc-bugzilla@gcc.gnu.org>
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This is the same testcase as PR26778. This bug is marked as resolved, and the
patch indeed prevents GCC from using useless mmx registers. Concerning integer
operations, the generated assembly got even better than GCC 3.4, as the values
are directly incremented in memory instead of being loaded and stored; no
callee-save register is used anymore. Unfortunately, there is still a
regression with respect to the floating-point stack.

Testcase compiled with -march=pentium3 -O3:

typedef union {
  long long l;
  double d;
} db_number;

double test(double x[3]) {
  double th = x[1] + x[2];
  if (x[2] != th - x[1]) {
    db_number thdb;
    thdb.d = th;
    thdb.l++;
    th = thdb.d;
  }
  return x[0] + th;
}

It may be clearer with a unified diff between the assembly code generated by
3.4.6 and the one by 4.2.0 (svn 2006-09-12). Note: the assembly code for 3.4
was edited by hand in order to reduce the noise due to mismatching integer
registers.

        pushl   %ebp
        movl    %esp, %ebp
        subl    $16, %esp
        movl    8(%ebp), %eax
        fldl    8(%eax)
        fldl    16(%eax)
        fld     %st(1)
        fadd    %st(1), %st
+       fstl    -8(%ebp)
        fsub    %st, %st(2)
        fxch    %st(1)
        fucomip %st(2), %st
        fstp    %st(1)
        jp      .L7
-       je      .L2
+       je      .L9
 .L7:
        fstpl   -16(%ebp)
        addl    $1, -16(%ebp)
        adcl    $0, -12(%ebp)
        fldl    -16(%ebp)
+       fstpl   -8(%ebp)
+       jmp     .L2
+       .p2align 4,,7
+.L9:
+       fstp    %st(0)
+       .p2align 4,,15
 .L2:
+       fldl    -8(%ebp)
        faddl   (%eax)
        leave
        ret

The 3.4 code never stores the value of th; it is kept at the top of the
floating-point stack. In my opinion, this is optimal. This is no longer the
case with 4.2 code. The value of "th" is stored in -8(%ebp). Then, on one
branch (L7), it is overwritten with the content of -16(%ebp). And on the other
branch (L9), the value is discarded from the top of the stack and then
immediatly (L2) reloaded from memory. Each line prefixed by + is useless: if
none is present, the code will still behave correctly and it will contain five
assembly instructions less.


-- 
           Summary: Useless floating-point stores and loads on x86
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: guillaume dot melquiond at ens-lyon dot fr


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29042