[Bug tree-optimization/85726] [6/7/8/9 Regression] div C1 to div C2 match.pd suboptimization

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85726

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
           Priority|P3                          |P2
   Target Milestone|---                         |6.5

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  On x86 the diff from GCC 4.8 to GCC 7 at -O2 is

@@ -6,8 +6,16 @@
 x:
 .LFB0:
        .cfi_startproc
+       movl    %edi, %eax
+       movl    $715827883, %edx
+       imull   %edx
+       movl    %edi, %eax
+       sarl    $31, %eax
+       sarl    $3, %edx
+       subl    %eax, %edx
        leal    15(%rdi), %eax
        testl   %edi, %edi
+       movl    %edx, ww(%rip)
        movl    $1431655766, %edx
        cmovs   %eax, %edi
        movl    %edi, %ecx
@@ -15,9 +23,9 @@
        sarl    $4, %ecx
        movl    %ecx, %eax
        imull   %edx
-       subl    %edi, %edx
-       leal    (%rdx,%rdx,2), %eax
-       movl    %edx, ww(%rip)
+       movl    %edx, %eax
+       subl    %edi, %eax
+       leal    (%rax,%rax,2), %eax
        subl    %eax, %ecx
        movl    %ecx, %eax
        ret