Bug 43137

Summary: redundant register move for sign extending
Product: gcc Reporter: Carrot <carrot>
Component: targetAssignee: Not yet assigned to anyone <unassigned>
Severity: enhancement CC: carrot, gcc-bugs
Priority: P3 Keywords: missed-optimization
Version: 4.5.0   
Target Milestone: ---   
Host: i686-linux Target: arm-eabi
Build: i686-linux Known to work:
Known to fail: Last reconfirmed: 2010-02-22 14:32:54

Description Carrot 2010-02-22 09:39:21 UTC
Compile following code with options -march=armv7-a -mthumb -Os

int foo();
long long bar22()
  int result = foo();
  return result;

GCC generates:

        push    {r3, lr}
        bl      foo
        asrs    r3, r0, #31    // A
        mov     r1, r3         // B
        pop     {r3, pc}

Instruction A,B can be simplified to "asrs r1, r0, #31".

In rtl expand, result was first extended to a DI register and then the DI register was moved to return register. In later passs the low half of the return value and variable result were recognized as in the same register, so the low part move was removed. After register allocation I get:

(call_insn 5 2 7 2 src/t5.c:5 (parallel [
            (set (reg:SI 0 r0)
                (call (mem:SI (symbol_ref:SI ("foo") [flags 0x41]  <function_decl 0x7f6e60fe0600 foo>) [0 S4 A32])
                    (const_int 0 [0x0])))
            (use (const_int 0 [0x0]))
            (clobber (reg:SI 14 lr))
        ]) 255 {*call_value_insn} (nil)

(insn 7 5 22 2 src/t5.c:5 (set (reg:DI 2 r2 [orig:136 result ] [136])
        (sign_extend:DI (reg/v:SI 0 r0 [orig:133 result ] [133]))) 691 {*thumb2_extendsidi2} (nil))

(insn 22 7 15 2 src/t5.c:7 (set (reg:SI 1 r1 [+4 ])
        (reg:SI 3 r3 [ result+4 ])) 658 {*thumb2_movsi_insn} (nil))

The high part register move didn't get any chance to be removed until the end of compilation.
Comment 1 Ramana Radhakrishnan 2010-02-22 14:32:54 UTC
Comment 2 Bernd Schmidt 2010-09-06 22:32:39 UTC
Subject: Bug 43137

Author: bernds
Date: Mon Sep  6 22:32:26 2010
New Revision: 163935

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=163935
	PR target/43137
	* config/arm/iterators.md (qhs_zextenddi_cond, qhs_sextenddi_cond):
	New define_mode_attrs.
	* config/arm/arm.md (zero_extendsidi2, arm_zero_extendsidi2,
	arm_exxtendsidi2, arm_extendsidi2): Delete patterns.
	(zero_extend<mode>di2, extend<mode>di2 and related splits): New.
	(thumb1_zero_extendhisi2): Remove code to handle LABEL_REFs.
	Remove pool_range attribute.
	(arm_zero_extendhisi2, arm_zero_extendhisi2_v6, arm_zero_extendqisi2,
	arm_zero_extendqisi2_v6, thumb1_zero_extendqisi2_v6): Remove
	pool_range and neg_pool_range attributes.
	* config/arm/thumb2.md (thumb2_zero_extendsidi2,
	thumb2_zero_extendhidi2, thumb2_zero_extendqidi2, thumb2_extendsidi2,
	thumb2_extendhidi2, thumb2_extendqidi2): Delete.

	PR target/43137
	* gcc.target/arm/pr43137.c: New test.


Comment 3 Bernd Schmidt 2010-09-07 11:16:45 UTC