[llvm] r211543 - [FastISel][X86] Optimize selects when the condition comes from a compare.

[Stephan Bergmann]

This commit smells like it broke things at least on X86_64 Linux.  Compiling

> void SB(signed char x) {
>     int n(x ? 0 : -1);
>     for (;;) {
>         fprintf(stderr,"\n\nx=%d n=%d\n\n",(int)x,(int)n);
>         return;
>     }
> }

with -O0 and -fsanitize=address and -fsanitize=undefined (that's at 
least how I stumbled across it) generates broken code for the

   int n(x ? 0 : -1)

line:

> 	movl	$0, %eax                                         # eax: 0
> 	movq	80(%rsp), %rcx          # 8-byte Reload          # rcx: &x
> 	cmpb	$0, (%rcx)                                       # z: x == 0
> 	setne	%dl                                              # dl: x != 0
> 	subl	$1, %eax                                         # eax: -1
> 	seto	%sil                                             # sil: 0
> 	xorb	$1, %sil                                         # sil: 1
> 	testb	$1, %sil                                         # z: 0
> 	movb	%dl, 69(%rsp)           # 1-byte Spill
> 	movl	%eax, 64(%rsp)          # 4-byte Spill
> 	jne	.LBB84_10                                        # taken
> # BB#9:                                 # %handler.negate_overflow
> .Ltmp7419:
> 	callq	__asan_handle_no_return at PLT
> 	leaq	.L__unnamed_1810(%rip), %rax
> 	movabsq	$1, %rsi
> 	movq	%rax, %rdi
> 	callq	__ubsan_handle_negate_overflow_abort at PLT
> .Ltmp7420:
> .LBB84_10:                              # %cont
> 	movl	$0, %eax
> .Ltmp7421:
> 	movb	63(%rsp), %cl           # 1-byte Reload
> 	cmpb	$0, %cl
> 	movl	64(%rsp), %edx          # 4-byte Reload
> 	cmovnel	%eax, %edx

While the 1-byte spill of x!=0 writes to 69(%rsp), its 1-byte reload 
(after the ubsan handler for the case that computing -1 = 0 - 1 would 
have overflown) erroneously reads from 63(%rsp).

If I locally revert all of r211543, r211544, r211545, r211645, the 
problem goes away (the instructions in the cont block are slightly 
different now, but correctly spill in 69(%rsp) instead of 63(%rsp)).

Stephan

On 06/23/2014 11:55 PM, Juergen Ributzka wrote:
> Author: ributzka
> Date: Mon Jun 23 16:55:36 2014
> New Revision: 211543
>
> URL: http://llvm.org/viewvc/llvm-project?rev=211543&view=rev
> Log:
> [FastISel][X86] Optimize selects when the condition comes from a compare.
>
> Optimize the select instructions sequence to use the EFLAGS directly from a
> compare when possible.