http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53352
Bug #: 53352 Summary: Incorrect CSE optimization on RTL expressions with a paradoxical subreg Classification: Unclassified Product: gcc Version: 4.8.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: rtl-optimization AssignedTo: unassig...@gcc.gnu.org ReportedBy: mead...@codesourcery.com Host: GNU/Linux Target: arm-none-linux-gnueabi The attached reproduction case exhibits what I believe to be a bug in the CSE pass. The problem can be reproduced from the mainline at r187470. The compiler was built for arm-none-linux-gnueabi and the reproduction case was compiled with -O3. Out of the gate the expanded RTL of interest looks like: (insn 12 11 13 3 (set (reg:SI 140) (lshiftrt:SI (reg/v:SI 135 [ tmp1 ]) (const_int 16 [0x10]))) (nil)) (insn 13 12 14 3 (set (reg:QI 141) (subreg:QI (reg:SI 140) 0)) (nil)) (insn 14 13 15 3 (set (reg:SI 142) (subreg:SI (reg:QI 141) 0)) (nil)) (insn 15 14 16 3 (set (reg:SI 134 [ tmp1$2 ]) (and:SI (reg:SI 142) (const_int 255 [0xff]))) (nil)) ... (insn 29 28 30 3 (set (reg:SI 0 r0) (const_int 0 [0])) (nil)) after "cse1" things look like: (insn 12 11 13 2 (set (reg:SI 140) (const_int 65280 [0xff00])) (nil)) (insn 13 12 14 2 (set (reg:QI 141) (subreg:QI (reg:SI 140) 0)) (expr_list:REG_EQUAL (const_int 0 [0]) (nil))) ;; This is *not* equal to zero because the upper ;; two bytes are undefined. (insn 14 13 15 2 (set (reg:SI 142) (subreg:SI (reg:QI 141) 0)) (expr_list:REG_EQUAL (const_int 0 [0]) (nil))) (insn 15 14 16 2 (set (reg:SI 134 [ tmp1$2 ]) (reg:SI 142)) (expr_list:REG_EQUAL (const_int 0 [0]) (nil))) ... (insn 29 28 30 2 (set (reg:SI 0 r0) (reg:SI 142)) (expr_list:REG_EQUAL (const_int 0 [0]) (nil))) I believe the REG_EQUAL note on the set involving a paradoxical subreg is incorrect. It eventually causes 0xFF00 to be passed to the function 'foo'. I doesn't look like 'equiv_constant' handles the paradoxical subreg case correctly. I fixed it as follows and am testing the patch now: Index: gcc/cse.c =================================================================== --- gcc/cse.c (revision 187470) +++ gcc/cse.c (working copy) @@ -3786,8 +3786,11 @@ } } - /* Otherwise see if we already have a constant for the inner REG. */ + /* Otherwise see if we already have a constant for the inner REG. + Don't bother with paradoxical subregs because we have no way + of knowing what the upper bytes are. */ if (REG_P (SUBREG_REG (x)) + && (GET_MODE_SIZE (mode) <= GET_MODE_SIZE (imode)) && (new_rtx = equiv_constant (SUBREG_REG (x))) != 0) return simplify_subreg (mode, new_rtx, imode, SUBREG_BYTE (x));