Souper says:

; Function: fn1
%0:i8 = var 
%1:i1 = slt 255:i8, %0
pc %1 1:i1
%2:i8 = sub 0:i8, %0
%3:i1 = slt 0:i8, %2
cand %3 0:i1

COMMAND: /home/regehr/souper/build/souper -stp-path=/usr/local/bin/stp reduce_378/foo.bc

LLVM says:

define void @fn1() #0 {
entry:
  %.pr = load i8* @a, align 1, !tbaa !1
  %cmp2 = icmp sgt i8 %.pr, -1
  br i1 %cmp2, label %for.body.lr.ph, label %for.end

for.body.lr.ph:                                   ; preds = %entry
  %0 = sub i8 0, %.pr
  %1 = icmp sgt i8 %0, 0
  %.op = add i8 %.pr, -1
  %2 = select i1 %1, i8 %.op, i8 -1
  store i32 0, i32* @b, align 4, !tbaa !4
  store i8 %2, i8* @a, align 1, !tbaa !1
  br label %for.end

for.end:                                          ; preds = %for.body.lr.ph, %entry
  ret void
}

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -c -w -emit-llvm -O3 reduce_378/foo.c -o reduce_378/foo.bc

C source code:

char a;
int b;
void fn1() {
  for (; a >= 0; a--)
    b = 0;
}

x86-64 from LLVM:

fn1:                                    # @fn1
	movb	a(%rip), %al
	testb	%al, %al
	js	.LBB0_5
	movb	%al, %cl
	negb	%cl
	testb	%cl, %cl
	jg	.LBB0_2
	movb	$-1, %al
	jmp	.LBB0_4
.LBB0_2:
	decb	%al
.LBB0_4:                                # %for.body.lr.ph
	movl	$0, b(%rip)
	movb	%al, a(%rip)
.LBB0_5:                                # %for.end
	retq

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -w -O3 reduce_378/foo.c -S -o -

x86-64 from GCC:

fn1:
	movzbl	a(%rip), %eax
	testb	%al, %al
	js	.L1
.L6:
	subb	$1, %al
	jns	.L6
	movl	$0, b(%rip)
	movb	%al, a(%rip)
.L1:
	rep ret

COMMAND: gcc -w -O3 reduce_378/foo.c -S -o -